Successively biaxial-oriented porous polypropylene film and process for production thereof

ABSTRACT

Disclosed is a successively biaxially stretched film obtained by successive biaxial stretching method comprising extruding a melt of a β-crystal nucleating agent-containing polypropylene-based resin composition from a T-die, cooling the extruded resin on a chill roll, and stretching the resulting web sheet longitudinally and then transversely, wherein the longitudinally stretched sheet is made to have a degree of β-crystal orientation of less than 0.3 by the following method (I) and/or (II), optionally subjected to annealing treatment, and transversely stretched:
         method (I): melting the polypropylene-based resin composition containing needle crystals of a specific β-crystal nucleating agent at a temperature not lower than m.p. of the polypropylene-based resin and lower than dissolution temperature of the β-crystal nucleating agent in the polypropylene-based resin melt, and extruding the melt from the T-die at the same temperature,   method (II): adjusting neck-in ratio during longitudinal stretching to 25 to 55%.       

     The porous polypropylene film has good breakage resistance during manufacture, excellent thickness uniformity, high porosity and air-permeability, and is useful for battery separators.

TECHNICAL FIELD

This invention relates to a successively biaxially stretchedpolypropylene porous film having numerous fine, continuousthrough-pores, and to a process for producing this film, and to abattery separator and so forth composed of this film.

BACKGROUND ART

Polypropylene occurs in crystal states such as α-crystals andβ-crystals, and β-crystals can be produced preferentially by employingspecial crystallization conditions or by adding a β-crystal nucleatingagent. β-Crystals are known to undergo a transition into stableα-crystals when subjected to thermal and dynamic action, and recentlyseveral methods have been proposed for producing an air-permeablepolypropylene film having continuous through-pores, which make use ofthe crystal transition that occurs in the course of stretching (JapaneseUnexamined Patent Publications H7-118429, H9-176352, H9-255804, andH6-100720). However, the pore formation mechanism involving β-crystalsis complicated, and is not yet fully understood. Consequently, thesemethods have failed to produce a porous film in a stable manner.

In order to obtain a porous film, all of the above publicationsrecommends to form β-crystals in a largest possible amount in theunstretched web sheet before stretching, and then carry out stretchingat an optimal temperature, wherein a K value determined by X-raydiffraction is employed as an index of the β-crystal content. A K valueof 1.0 indicates a β-crystal content of 100%, and with the understandingthat the higher the K value is, the easier it is to obtain a porous filmwith high air-permeability, it is recommended in Japanese UnexaminedPatent Publication H9-255804, for instance, that the K value be at least0.7, preferably 0.8 to 0.98. The recommended stretching temperature isabout 50 to 100° C. during longitudinal stretching, and about 100 to150° C. during transverse stretching.

These recommended K values are achieved relatively easily by adding aspecific β-crystal nucleating agent, without having to employ specialcrystallization conditions. However, stretching a sheet having a high Kvalue at the recommended temperature does not necessarily give a porousfilm with high permeability.

For example, the strain rate during stretching affects pore formation,and there is a strong tendency for pore formation to be impaired if thestrain rate is high during transverse stretching in particular. No poresmay sometimes be formed, if stretching is carried out at a transversestretching strain rate of at least 60 times/ minute (or 100%/second)which is usually used in the manufacture of an ordinary nonporous,biaxially stretched polypropylene film. The strain rate is determined asthe ratio V/D (or 100V %/D) of the stretching rate V to the sampledimension D in the stretching direction, and an extremely slow strainrate of less than 10 times/minute (17%/second) (longitudinal andtransverse directions) is recommended as a condition for forming poresin Japanese Unexamined Patent Publication H6-100720. However, decreasingthe strain rate is undesirable because it leads to lower productivity.

There are also cases in which no pores are formed even if the K value ishigh and preferable stretching temperature and slow strain rate areemployed. The mechanism by which pores are formed through β-crystals iscomplicated, and stable industrial manufacturing conditions had to beestablished, with plenty of room remaining for improvement.

Aside from the problem of pore formation, another serious problem up tonow has been the breakage of a film during its manufacture. Thisbreakage is ameliorated by utilizing β-crystals, as compared to a porousfilm containing a filler such as calcium carbonate, but the results arestill not satisfactory, and further improvement is needed.

In recent years, porous polypropylene films have been used in a widevariety of fields depending on the characteristics thereof.Specifically, they found use in disposable diapers, feminine sanitaryproducts and packaging materials due to their water vapor permeability,in synthetic paper and wallpaper materials due to their printingcharacteristics, and in filtration membrane and battery separators dueto their separation characteristics.

Their use as battery separators has been especially popular because thedemand for electronic devices has recently soared. Examples of theapplication of a porous polypropylene film containing β-crystals havebeen proposed in several places, such as Japanese Unexamined PatentPublication 2000-30683.

One of the most important properties of a battery separator is itselectrical resistance. Electrical resistance is the measured value ofthe resistance to current flowing through a separator between a cathodeand an anode, and is known to be proportional to the product of theGurley air-permeability and the pore size (“Kagaku Kogyo ” [ChemicalIndustry], January issue (1997) or R. W. Callahan et al., The TenthInternational Seminar on Primary and Secondary Battery Technology andApplication, Mar. 1–4, 1993). It is generally preferable that thiselectrical resistance be as low as possible, and in more specific terms,the electrical resistance per mil (25 μm) of thickness is preferablyless than 30 ohm·in, more preferably less than 20 ohm·in.

Japanese Unexamined Patent Publication 2000-30683 discloses severalrecommended stretching conditions, including the temperature and stretchratio during longitudinal and transverse stretching, and the totalstretching range. Nevertheless, a battery separator comprising theporous film disclosed in Japanese Unexamined Patent Publication2000-30683 is not necessarily satisfactory in terms of thicknessuniformity, and even if the recommended stretching conditions given inthis publication are employed, the resulting porous film did notnecessarily have the electrical resistance required of a batteryseparator.

Therefore, there has been a need to better understand the pore formationmechanism, and to establish an industrially optimal manufacturing methodthat is suited to this mechanism. In particular, the thicknessuniformity of a porous film was inadequate in the past, and consequentlythere was unsatisfactory uniformity in film characteristics, such asair-permeability, tensile characteristics, electrical resistance andporosity, and there was variance in the manufactured film from place toplace. Therefore, there is a need for the development of a porous filmwith superior thickness uniformity, as well as a process for producingsuch a film.

It is an object of the present invention to solve these problems, and inparticular to provide a porous polypropylene film which has goodthickness uniformity and high porosity and air-permeability, and whichpreferably has the electrical resistance required of a batteryseparator.

It is a further object of the present invention to provide amanufacturing process for preparing a porous polypropylene film which isresistant to breakage during manufacture, by which the film can bemanufactured in a stable manner and at a high strain rate.

DISCLOSURE OF THE INVENTION

We conducted extensive research in light of the above situation.Consequently, we discovered that if the β-crystal lamella layers of alongitudinally stretched sheet is oriented in a specific direction bythe following method (I) and/or method (II), pore formation is promotedin the subsequent transverse stretching step, with the result that thethickness uniformity in the biaxially stretched film that is finallyobtained is improved, and the resulting porous polypropylene film hashigh air-permeability and porosity, good feeling and electricalresistance required for a battery separator.

Method (I): an amide compound which is a β-crystal nucleating agent ismade into needle crystals, and during extrusion of a polypropylene-basedresin composition containing these needle crystals from a T-die, theresin temperature is set to be over the melting point of thepolypropylene-based resin and below the temperature at which the amidecompound dissolves in the polypropylene-based resin melt, and the meltof the polypropylene-based resin composition is extruded from a T-die ina state in which the needle crystals of the amide compound are present.

Method (II): the neck-in ratio during longitudinal stretching isadjusted to at least 25% and not more than 55%.

Japanese Unexamined Patent Publication H8-197640 proposes a method inwhich a polypropylene-based resin composition containing needle crystalsof this amide compound is extruded at a resin temperature that is abovethe melting point of the polypropylene-based resin and below thetemperature at which the amide compound dissolves in thepolypropylene-based resin melt, thereby orienting the crystal lamellalayers. This procedure is described to improve rigidity and heatdeformation temperature of the polypropylene-based resin molded product.However, this publication does not teach at all that this procedure isused for producing a polypropylene unstretched web sheet for producing astretched film, and that pore formation is promoted when thisunstretched web sheet is stretched.

Also, with a conventional process for producing a biaxially stretchedfilm, such as described in “Kobunshi Kako One Point (Hints forMacromolecular Processing), Vol. 2 “Film wo tsukuru (Making Films),””published on Oct. 5, 1988 by Kyoritsu Shuppan, page 48, the neck-inratio is usually kept as low as possible in the longitudinal stretchingstep for the sake of film uniformity, and therefore it was surprisingthat increase in the neck-in ratio as mentioned above orients theβ-crystal lamella layers and promote pore formation.

It has also been discovered that the provision of such a longitudinallystretched sheet in which the β-crystal lamella layer has been orientedin a specific direction by the above-mentioned method (I) and/or method(II) has the effect of making the film less prone to breakage during thetransverse stretching step, and increasing the transverse stretchingstrain rate.

Furthermore, research by the inventors has revealed that if an annealingtreatment is performed under specific conditions between thelongitudinal stretching step and the transverse stretching step, poreformation is further promoted, and the properties of the resultingbiaxially stretched film are further improved, and the strain rateduring transverse stretching can be even higher without impairingbreaking resistance.

The present invention has been accomplished on the basis of thesefindings, and particularly provides the following porous polypropylenefilm, a process for producing the film, and a battery separator.

Item 1. A successively biaxially stretched, β-crystal nucleatingagent-containing polypropylene porous film, comprising apolypropylene-based resin and a β-crystal nucleating agent, the filmhaving a film thickness uniformity of 0.1 or less, and the filmexhibiting the following pore structures (a) and (b) when observed incross section in the longitudinal and transverse directions of the filmunder an electron microscope:

(a) the cross section in the transverse direction: more lamella crosssections are present than in the image of a cross sectional in thelongitudinal direction; there are numerous pores between these lamellacross sections; the maximum pore size in the thickness direction of thepore is 0.1 to 5 μm and the maximum pore size in the transversedirection is 1 to 50 μm, and the ratio of the maximum pore size in thethickness direction/the maximum pore size in the transverse direction isfrom 1/2 to 1/20;

(b) the cross section in the longitudinal direction: there are nolamella cross sections or fewer lamella cross sections than in the imageof the cross section in the transverse direction; there are numerouspores; the maximum pore size in the thickness direction of the pores is0.1 to 5 μm, and the maximum pore size in the longitudinal direction is1 to 50 μm, and the ratio of the maximum pore size in the thicknessdirection/the maximum pore size in the longitudinal direction is from1/2 to 1/20.

Item 2. The successively biaxially stretched, β-crystal nucleatingagent-containing polypropylene porous film comprising thepolypropylene-based resin and the β-crystal nucleating agent, accordingto item 1 above, which has a film thickness uniformity of 0.07 to 0.04.

Item 3. The successively biaxially stretched, β-crystal nucleatingagent-containing polypropylene porous film according to item 1 or 2above, which has a Gurley air-permeability measured according to ASTMD726 of 10 to 100 sec/10 ml, and a porosity of 30 to 65%.

Item 4. The successively biaxially stretched, β-crystal nucleatingagent-containing polypropylene porous film according to any of items 1to 3 above, which has an estimated electrical resistance R of less than30 ohm·in/mil, as calculated according to the following equation fromthe Gurley air-permeability and the average pore size:R=25(4.2t _(Gur) d)/Lwherein R is the estimated electrical resistance (ohm·in/mil) of thefilm in a 31 wt % KOH solution, t_(Gur) is the Gurley air-permeability(sec/10 ml) measured according to ASTM D726, d is the average pore size(μm) determined by mercury intrusion porosimetry, and L is the filmthickness (μm).

Item 5. The successively biaxially stretched, β-crystal nucleatingagent-containing polypropylene porous film according to any one of items1 to 4 above, which has

an average pore size of 0.04 to 0.06 μm when measured by bubble pointmethod (JIS K 3832), and of 0.10 to 0.50 μm when measured by mercuryintrusion porosimetry,

a maximum pore size in the film thickness direction of 0.1 to 5 μm, anda maximum pore size in the direction perpendicular to the thicknessdirection of 1 to 50 μm, as determined from electron microscopy (SEM) ofa film cross sections,

a water vapor permeability of 3000 to 6000 g/m²·24 h as measuredaccording to JIS Z 0208,

a tensile strength according to JIS K 7127 of 50 to 100 MPa in both thelongitudinal and transverse directions, and

a water pressure resistance of 200 to 400 kPa as measured according toJIS L 1092 except that a 0.25 wt % aqueous solution of a surfactant(sodium polyoxyethylene lauryl ether sulfate (number of moles ofethylene oxide added=3 moles)) is used instead of pure water.

Item 6. The successively biaxially stretched, β-crystal nucleatingagent-containing polypropylene porous film according to any one of items1 to 5 above, wherein the β-crystal nucleating agent is:

(1) at least one member selected from the group consisting ofN,N′-diphenylhexanediamide, N,N′-dicyclohexylterephthalamide andN,N′-dicyclohexyl2,6-naphthalenedicarboxamide,

(2) at least one member selected from the group consisting ofN,N′-dicyclohexanecarbonyl-p-phenylenediamine,N,N′-dibenzoyl-1,5-diaminonaphthalene,N,N′-dibenzoyl-1,4-diaminocyclohexane andN,N′-dicyclohexanecarbonyl-1,4-diaminocyclohexane,

(3) at least one member selected from the group consisting ofN-cyclohexyl-4-(N-cyclohexanecarbonyl-amino)benzamide andN-phenyl-5-(N-benzoylamino)pentaneamide, or

(4) a mixture of at least two members of the above amide compounds of(1) to (3).

Item 7. A process for producing the successively biaxially stretched,β-crystal nucleating agent-containing polypropylene porous filmaccording to item 1 above, by a sequential biaxial stretching step whichcomprises extruding a melt of a polypropylene-based resin compositioncontaining a β-crystal nucleating agent and a polypropylene-based resinfrom a T-die, cooling the extrudate on a chill roll, and stretching thethus obtained β-crystal nucleating agent-containing polypropyleneunstretched web sheet first longitudinally and then transversely,characterized in that the degree of orientation of β-crystals calculatedfrom a pole figure of the crystal lattice (300) plane of the β-crystalsdetermined by X-ray diffraction of the sheet obtained after longitudinalstretching is adjusted to less than 0.30 by carrying out the followingmethod (I) and/or method (II):

method (I): providing a polypropylene-based resin composition containinga polypropylene-based resin and needle crystals of the amide compoundaccording to item 6 above as a β-crystal nucleating agent, melting thepolypropylene-based resin composition containing the needle crystals ofthe β-crystal nucleating agent at a temperature (T1) which is not lowerthan the melting point of the polypropylene-based resin and lower thanthe temperature at which the needle crystals of the amide compounddissolve in the melt of the polypropylene-based resin, and extrudingfrom the T-die the molten polypropylene-based resin composition at saidtemperature (T1) in a state in which the amide compound needle crystalsare present,

method (II): adjusting the neck-in ratio during longitudinal stretchingto at least 25% and not more than 55%.

Item 8. The process for producing a porous film according to item 7above, wherein the sheet after the longitudinal stretching is annealedat 130 to 160° C. for 1 to 300 seconds while being stretched in thelongitudinal direction at a longitudinal stretch ratio of 0 to 30%, andis then transversely stretched.

Item 9. The process for producing a porous film according to item 7 or 8above, wherein the stretching temperature is 120 to 155° C. and thestretch ratio is 4 to 10 times in the transverse stretching step, andthe transverse stretching is performed at a transverse stretching strainrate of 100 to 300%/sec.

Item 10. A battery separator comprising the successively biaxiallystretched, β-crystal nucleating agent-containing polypropylene porousfilm according to item 4 above.

Item 11. The process for producing a porous film according to item 7above, wherein the degree of orientation of the β-crystals is set toless than 0.28 by adjusting the neck-in ratio to at least 35% and notmore than 55% in the above-mentioned method (II).

Item 12. The process for producing a porous film according to item 7above, wherein the degree of orientation of the β-crystals is set toless than 0.27 by adjusting the neck-in ratio to at least 40% and notmore than 55% in the above-mentioned method (II).

Item 13. The process for producing a porous film according to item 7above, wherein the sheet after the longitudinal stretching is annealedat 140 to 150° C. for 1 to 60 seconds while being stretched in thelongitudinal direction at a transverse stretch ratio of 0 to 20%, and isthen transversely stretched.

Item 14. The process for producing a porous film according to item 7above, wherein the sheet after the longitudinal stretching is annealedat 145 to 150° C. for 1 to 10 seconds while being stretched in thelongitudinal direction at a longitudinal stretch ratio of 0 to 10%, andis then transversely stretched.

Item 15. The process for producing a porous film according to item 7above, wherein the unstretched web sheet has a β-crystal content of 60to 90%.

Item 16. The process for producing a porous film according to item 7above, wherein, in method (II), the unstretched web sheet is obtained bymelting pellets obtained from the β-crystal nucleating agent and thepolypropylene-based resin, extruding the resulting molten resin having atemperature of 200 to 280° C. from a T-die, and cooling andcrystallizing the obtained molten sheet at 110 to 130° C. for 10 to 60seconds.

Item 17. The battery separator according to item 10 above, wherein theβ-crystal nucleating agent is:

(1) at least one member selected from the group consisting ofN,N′-diphenylhexanediamide, N,N′-dicyclohexylterephthalamide andN,N′-dicyclohexyl2,6-naphthalenedicarboxamide,

(2) at least one member selected from the group consisting ofN,N′-dicyclohexanecarbonyl-p-phenylenediamine,N,N′-dibenzoyl-1,5-diaminonaphthalene,N,N′-dibenzoyl-1,4-diaminocyclohexane andN,N′-dicyclohexanecarbonyl-1,4-diaminocyclohexane,

(3) at least one member selected from the group consisting ofN-cyclohexyl-4-(N-cyclohexanecarbonyl-amino)benzamide andN-phenyl-5-(N-benzoylamino)pentaneamide, or

(4) a mixture of at least two members of the above amide compounds of(1) to (3).

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 shows X-ray diffraction diagrams of longitudinally stretchedsheets obtained at neck-in ratios of 15% and 45%.

FIG. 2 is a conceptual diagram of the orientation of the β-crystallamellas in a longitudinally stretched sheet.

FIG. 3 shows electron micrographs (SEM; 1000× magnification) of thecross section of the porous biaxially stretched film of the presentinvention obtained by transversely stretching a longitudinally stretchedsheet obtained at a neck-in ratio of 45%, with (A) being an image of across section in the transverse direction (TD) of the porous biaxiallystretched film, and (B) being an image of a cross section in thelongitudinal direction (MD).

FIG. 4 is a conceptual diagram illustrating the structure of a crosssection of the porous biaxially stretched film of the present inventionobtained by transversely stretching a longitudinally stretched sheetobtained at a neck-in ratio of 45%.

FIGS. 5 (i) and (ii) show micrographs of an unstretched web sheet (priorto longitudinal stretching) prepared according to Example A (in which aporous film was prepared by method (I) of the present invention) andExample 1 (in which a porous film was manufactured by method (II),without employing method (I) of the present invention), respectively.

FIG. 6 is a conceptual diagram illustrating the steps of producing aporous film by methods (I) and (II) of the present invention.

In the drawings, the symbols have the following meanings.

-   1 β-crystal lamella-   2 lamella cross section-   3 pore-   4 stretched portion-   Xt maximum pore size in the transverse direction-   Xm maximum pore size in the longitudinal direction-   Y maximum pore size in the thickness direction-   11 p columnar crystals of β-crystal nucleating agent-   11 n needle crystals of β-crystal nucleating agent-   22 solidified polypropylene-based resin-   23 molten polypropylene-based resin-   24 β-crystal lamella of polypropylene-based resin-   31 pore

DETAILED DESCRIPTION OF THE INVENTION

The present invention is characterized in that the degree of orientationof β-crystals calculated from a pole figure of the crystal lattice (300)plane of the β-crystals determined by X-ray diffraction of a sheetobtained after longitudinal stretching is adjusted to less than 0.30. Inthe present invention, this adjustment of the degree of orientation toless than 0.30 is accomplished either by performing the above-mentionedmethod (I) or method (II), or by combining methods (I) and (II).

Method (I) of the present invention involves longitudinally stretchingand transversely stretching an unstretched web sheet in which β-crystalsof a polypropylene-based resin have been oriented. Specifically, apolypropylene-based resin composition in which needle crystals of anamide compound which is a β-crystal nucleating agent have precipitatedis extruded from a T-die at a temperature which is lower than thetemperature at which the needle crystals of the amide compound dissolvein the molten polypropylene-based resin, whereby the needle crystals areoriented. The thus-obtained extruded resin from the T-die is cooled,whereby an unstretched web sheet is obtained in which the β-crystals ofthe polypropylene-based resin have crystallized as oriented along theneedle crystals of the above-mentioned amide compound. When theunstretched web sheet is longitudinally stretched, the degree oforientation of the β-crystals in the longitudinally stretched sheet isless than 0.30.

Method (II) of the present invention involves adjusting the neck-inratio during the longitudinal stretching of the unstretched web sheet.When an unstretched web sheet obtained by an ordinary method isstretched longitudinally, the unstretched web sheet shrinks in its widthdirection, that is, transversely, and the sheet width decreases. The“neck-in ratio” in the present invention refers to this shrinkage.Method (II) of the present invention is characterized in that theneck-in ratio in the longitudinal stretching step is at least 25%,preferably at least 35%, more preferably at least 40% and not more than55%. With an increase in the neck-in ratio, the orientation of theβ-crystal lamella layers increases, and the above-mentioned degree oforientation of the β-crystals in the longitudinally stretched sheetbecomes less than 0.30.

The longitudinally stretched sheet obtained by the above-mentionedmethod (I) and/or method (II) is then subjected to transversestretching, whereby pore formation is promoted, so that pore formationproceeds smoothly even at a high strain rate, and a porous film withhigh air-permeability is obtained.

Successively Biaxially Stretched Polypropylene Porous Film

As mentioned above, the sequentially biaxially stretched polypropyleneporous film of the present invention contains a polypropylene-basedresin and a β-crystal nucleating agent, and has excellent thicknessuniformity. This sequentially biaxially stretched β-crystal nucleatingagent-containing polypropylene porous film exhibits the following porestructure (a) and (b) when observed in cross sections in thelongitudinal and transverse directions of the film under an electronmicroscope.

(a) cross section in the transverse direction: more lamella crosssections are present than in the image of the cross section in thelongitudinal direction; there are numerous pores between these lamellacross sections; the maximum pore size in the thickness direction of thepores is 0.1 to 5 μm, the maximum pore size in the transverse directionis 1 to 50 μm, and the ratio of the maximum pore size in the thicknessdirection/the maximum pore size in the transverse direction is from 1/2to 1/20,

(b) cross section in the longitudinal direction: there are no lamellacross sections or fewer lamella cross sections than in the image of thecross section in the transverse direction, there are numerous pores, themaximum pore size in the thickness direction of the pores is 0.1 to 5μm, the maximum pore size in the longitudinal direction is 1 to 50 μm,and the ratio of the maximum pore size in the thickness direction/themaximum pore size in the longitudinal direction is from 1/2 to 1/20.

The reason the film of the present invention has the above-mentionedpore structure is not yet fully clarified, but is surmised to be asfollows.

As mentioned above, method (I) and/or method (II) results in thelongitudinal orientation of the β-crystal lamella layers in thelongitudinally stretched sheet upon completion of the longitudinalstretching. More specifically, the degree of orientation of β-crystalscalculated from a pole figure of the crystal lattice (300) plane of theβ-crystals determined by X-ray diffraction of the sheet obtained afterlongitudinal stretching is less than 0.30. When the longitudinallystretched sheet comprising lamella layers stacked in the width directionas a result of their longitudinal orientation is then stretchedtransversely, the stacked lamella layers are pulled apart, forming poresbetween the lamella layers, and this is believed to result in thestructures described in (a) and (b) above.

This point will now be described in further detail first on the basis ofmethod (II) in which the neck-in ratio during longitudinal stretching isat least 25% and not more than 55%.

FIG. 1 shows X-ray diffraction images of longitudinally stretched sheetsobtained at neck-in ratios of 15% and 45%. The longitudinal stretchingwas performed using a β-crystal unstretched web sheet with a K value of0.96, at a stretching temperature of 90° C. and a stretch ratio of 4times. The X-ray diffraction measurements were made in the sheetthickness direction (“THROUGH” direction), sheet width direction (“EDGE”direction), and longitudinal direction (“END” direction).

Comparison of the diffraction images in the EDGE direction reveals thatthe diffraction peak of the crystal lattice plane (300) originating inβ-crystals that appears at a neck-in ratio of 15% disappears at aneck-in ratio of 45%. This indicates that the orientation of β-crystallamellas is higher at 45% than at 15%.

Specifically, when an unstretched web sheet is longitudinally stretched,the β-crystal lamellas are oriented so as to be stacked in the sheetwidth direction, with part of them making a transition into α-crystals.The neck-in ratio here affects the orientation of the β-crystal lamellalayers, and it is surmised that the orientation of β-crystals increaseswith an increase in the neck-in ratio. FIG. 2 shows a conceptual diagramof this orientation of β-crystal lamella layers.

At a neck-in ratio of 45%, the orientation of crystal lamellas 1 shownin FIG. 2 increases more than that achieved at a neck-in ratio of 15%,and this is believed to be why the β-crystal (300) plane diffractionpeak disappears in the EDGE direction. On the other hand, it is surmisedthat the (300) plane diffraction peaks were detected in the threedifferent measurement directions because the orientation of theβ-crystal lamellas 1 is inadequate at a neck-in ratio of 15%.

FIG. 3 shows micrographs (SEM; 1000× magnification) of the cross sectionof a porous biaxially stretched film obtained by transversely stretchinga longitudinally stretched sheet obtained at a neck-in ratio of 45%, inwhich the above-mentioned lamella layers 1 are oriented (the porousbiaxially stretched film obtained in Example 1 to be described later),and FIG. 4 is a conceptual diagram of the same film. FIG. 3 (A) shows across section in the transverse direction (TD) of this porous biaxiallystretched film, and FIG. 3 (B) shows a cross section in the longitudinaldirection (MD) of this porous biaxially stretched film.

Because more lamella cross sections are observed in the cross section inthe transverse direction (TD)(TD cross section) than in the crosssection in the longitudinal direction (MD) (MD cross section), it issurmised that in the present invention the lamella layers are pulledapart in the transverse stretching step, with the result that pores areformed.

Referring to FIG. 4, there are more lamella cross sections 2 in theimage of the cross section in the transverse direction (TD crosssection) than in the cross section in the longitudinal direction (MDcross section), and there are numerous pores 3 between these lamellacross sections, and the maximum pore size in the thickness direction (Y)and the maximum pore size in the transverse direction (Xt) of thesepores are 0.1 to 5 μm and 1 to 50 μm, respectively, and the ratio of themaximum pore size in the thickness direction (Y)/the maximum pore sizein the transverse direction (Xt) is 1/2 to 1/20.

In the cross section in the longitudinal direction (MD cross section) inFIG. 4, there are no lamella cross sections or fewer lamella crosssections than in the transverse cross section image (TD cross sectionimage). There are numerous pores 3, and the maximum pore size in thethickness direction (Y) and the maximum pore size in the longitudinaldirection (Xm) of the pores are 0.1 to 5 μm and 1 to 50 μm,respectively, and the ratio of the maximum pore size in the thicknessdirection (Y)/the maximum pore size in the longitudinal direction (Xm)is 1/2 to 1/20.

The above-mentioned maximum pore size in the transverse direction (Xt),maximum pore size in the longitudinal direction (Xm), and maximum poresize in the thickness direction (Y) were measured by the methods givenin item “Pore size” in the Examples to be described later. For themaximum pore size in the thickness direction (Y), a cross section in thetransverse direction (TD cross section) and a cross section in thelongitudinal direction (MD cross section) were both observed, and thispore size was determined for the pores with the largest pore size in thethickness direction.

Biaxially stretched films obtained by transversely stretchinglongitudinally stretched sheets with the above-mentioned neck-in ratiosof 15% and 45%, respectively, at 140° C. and a ratio of 6.0 times weremeasured for Gurley air-permeability (sec/10 mL) according to ASTM D726,which was found to be 100 (Comparative Example 1 to be described later)and 12 (Example 1 to be described later), respectively, with the latterexhibiting a higher air-permeability than the former. This is surmisedto be because the increase in the orientation of the lamella layerspromoted pore formation.

On the other hand, method (I) involves orienting the β-crystal lamellalayers in a step prior to the longitudinal stretching, that is, duringthe manufacture of the unstretched web sheet, and produces the sameeffect as when the neck-in ratio was increased to 45% in method (II)above.

Specifically, when the amide compound according to item 6 above, whichis a β-crystal nucleating agent, is completely dissolved in a moltenpolypropylene-based resin and then cooled, the amide compoundrecrystallizes within the polypropylene-based resin, forming needlecrystals, and when a polypropylene-based resin composition containingthese needle crystals is extruded from a T-die at a temperature that isnot lower than the melting point of polypropylene and lower than thetemperature at which the amide compound dissolves in thepolypropylene-based resin melt, shear force orients the needle crystalsof this amide compound in the direction of resin flow. The needlecrystals thus oriented serve as crystal nuclei for the crystallizationof the polypropylene-based resin into β-crystals. The β-crystal lamellalayers may already be oriented at the point the unstretched web sheet isobtained, and the degree of orientation may be less than 0.30 in somecases, but longitudinally stretching the unstretched web sheet in theusual way further raises the degree of orientation of the β-crystallamella layers so that the degree of orientation of β-crystal lamellalayers in the longitudinally stretched sheet is less than 0.30. It isbelieved that as a result, the β-crystal lamella layers are oriented soas to be stacked in the sheet width direction, resulting in the samestate of orientation as when the neck-in ratio is increased.

Therefore, when method (I) is employed, the neck-in ratio does notnecessarily have to be raised in the subsequent longitudinal stretchingstep as in method (II). Still, combining methods (I) and (II) makes itpossible to further raise the degree of orientation of the β-crystallamella layers, and to promote pore formation to the maximum.

The polypropylene-based resin composition used in method (I) containsneedle crystals of the amide compound described in item 6 above, whichis a β-crystal nucleating agent. This polypropylene-based resincomposition is prepared as follows. The amide compound is added to apolypropylene-based resin, and then melt kneading is performed above thetemperature at which the amide compound dissolves in thepolypropylene-based resin melt, so as to homogeneously dissolve theamide compound in the polypropylene-based resin melt. When this moltenresin is cooled, the amide compound precipitates as needle crystals inthe polypropylene-based resin. The crystal state of the amide compoundprior to the melt kneading is usually that of columnar crystals, butwhen these are homogeneously dissolved in the polypropylene-based resinmelt and then cooled, the crystal form changes into needle form.Therefore, if the melt kneading temperature is below the temperature atwhich the amide compound dissolves in the polypropylene-based resinmelt, no needle crystals are formed. If the amide compound remains inthe form of columnar crystals, there is no increase in the degree oforientation of the β-crystal lamella layers in the subsequent T-dieextrusion and crystallization steps.

FIGS. 5 (i) and (ii) show micrographs of an unstretched web sheet (priorto longitudinal stretching) prepared according to Example A (in which aporous film was manufactured by method (I) of the present invention) andExample 1 (in which a porous film was manufactured by method (II),without employing method (I) of the present invention), respectively.These micrographs were taken in a state in which the polypropylene-basedresin had been melted on a 200° C. hot plate.

It can be seen from FIG. 5 (i) that in the unstretched web sheetprepared using method (I), the needle crystals of the amide compound areoriented in the direction of resin flow (MD direction). On the otherhand, it can be seen from FIG. 5 (ii) that in the unstretched web sheetprepared without using method (I), there is no distinct orientation ofthe columnar crystals of the amide compound in the direction of resinflow (MD direction).

The process of manufacturing a porous film by the above-mentionedmethods (I) and (II) is believed to be as shown in the conceptualdiagram of FIG. 6.

Specifically, with method (I), as shown under “Method (I)” in FIG. 6,pellets are first obtained which contain a solid polypropylene-basedresin and needle crystals 11 n of the above-mentioned β-crystalnucleating agent (I-1), and these pellets are melted at a temperature(T1) that is not lower than the melting point of the polypropylene-basedresin and lower than the temperature at which the amide compound needlecrystals dissolve in the polypropylene-based resin, thereby giving amolten resin composition comprising the above-mentioned needle crystals11 n and molten polypropylene-based resin 23 (I-2). The molten resincomposition thus obtained is extruded from a T-die at theabove-mentioned temperature (T1) in a state in which the amide compoundneedle crystals 11 n are present, whereupon the needle crystals 11 n areoriented along the flow of resin, and the extruded molten sheet containsthe needle crystals 11 n as oriented (I-3). When the molten sheet iscooled on a chill roll, the polypropylene-based resin crystallizes alongthe needle crystals 11 n, so that an unstretched web sheet is obtainedin which β-crystal lamellas 24 of the polypropylene-based resin arepresent in an oriented state (I-4). This unstretched web sheet is thenlongitudinally stretched, and this further raise the degree oforientation of the β-crystal lamellas 24 (I-5). Transverse stretching isthen performed to produce pores 31 between the β-crystal lamellas,giving a porous film (I-6).

On the other hand, with method (II), as shown under “Method (II)” inFIG. 6, pellets are first obtained which comprise a solidpolypropylene-based resin and columnar crystals (which may be needlecrystals) 11 p of a β-crystal nucleating agent (II-1). These pellets arethen melted, although the temperature conditions here are not important.

When these pellets are melted at a temperature (T2) that is not lowerthan the melting point of the polypropylene-based resin and lower thanthe temperature at which the crystals of b-crystal nucleating agentdissolve in the polypropylene-based resin melt, a molten resincomposition is obtained which comprises a molten polypropylene-basedresin 23 and the above-mentioned columnar crystals 11 p (II-2). Themolten resin composition thus obtained is extruded from a T-die at theabove-mentioned temperature (T2) in a state in which the amide compoundcolumnar crystals 11 p are present, whereupon the columnar crystals 11 pare slightly oriented along the flow of resin, although the degree oforientation is lower than in the case of needle crystals, and thecolumnar crystals 11 p are contained in an unoriented state in theextruded molten sheet (II-3). When the melt is cooled on a chill roll,the polypropylene-based resin crystallizes along the columnar crystals11 p, with the result that an unstretched web sheet is obtained in whichb-crystal lamellas 24 of the polypropylene-based resin are present in anunoriented state (II-4).

Alternatively, when the above-mentioned pellets are melted at atemperature (T3) (II-2a), which is not lower than the melting point ofthe polypropylene-based resin and not lower than the temperature atwhich the crystal nucleating agent crystals dissolve in thepolypropylene-based resin, and then extruded from a T-die at the sametemperature (T3) (II-3a), needle crystals of the β-crystal nucleatingagent precipitate in an unoriented state in the course of cooling andcrystallization on a chill roll, with the result that an unstretched websheet is obtained in which β-crystal lamellas 24 of thepolypropylene-based-resin, crystallized along the precipitated β-crystalnucleating agent crystals, are present in an unoriented state (II-4a).

The unstretched web sheet in which the β-crystal lamellas 24 of thepolypropylene-based resin are present in an unoriented state is thenlongitudinally stretched at a high neck-in ratio of 25 to 55%, wherebythese β-crystal lamellas 24 are oriented (II-5). Then transversestretching of this product forms the pores 31 between the β-crystallamellas, giving a porous film (II-6).

If needed, with the present invention, the longitudinally stretchedsheet can be annealed under specific conditions after longitudinalstretching but prior to transverse stretching. This further promotespore formation in the subsequent transverse stretching, and improves theporosity and air-permeability. In this annealing, it seems that some orall of the β-crystals undergo crystal transition to α-crystal lamellalayers, while the degree of orientation of the β-crystal lamella layersis maintained, and that this change in crystal form further promotespore formation, but the details are not clear.

The porous polypropylene film of the present invention having the abovepore structure not only has excellent air-permeability and water vaporpermeability, it also has excellent leakage resistance and mechanicalstrength. Accordingly, the film of the present invention can be used ina wide range of fields, such as light rain wear, light work clothes andother moisture-permeable waterproof garments, hygienic products (such asdiapers (including disposable diapers and pants-shaped diapers),sanitary napkins and other such feminine products, incontinence pads andother such absorbent articles, bed sheets and other hygienicmerchandise), waterproof sheets, wallpaper and other constructionmaterials, packaging materials for desiccants, deoxygenators, chemicalhand warmers and the like, synthetic paper, filtration membranes andseparation membranes, medical materials, agricultural multi-sheets, andbattery separators used in batteries, electrolysis and so forth.

In particular, the porous polypropylene film of the present inventionhas very good thickness uniformity. The thickness uniformity of the filmis 0.1 or less, particularly 0.1 to 0.04, preferably 0.07 to 0.04.Because the film of the present invention has excellent uniformity inits film characteristics, such as air-permeability, tensile strength,electrical resistance and porosity, there is substantially no variancein these properties from place to place of the film, and this is alsoadvantageous in terms of production stability.

The term “thickness uniformity of the film” as used in the presentinvention refers to the following. The thickness of the obtained porousfilm was measured at 100 points, with a 1 cm separation in thelongitudinal direction, along the center line in the width direction ofthe film (that is, the center line longitudinally connecting points thatdivide the film width into two equal halves), the average thickness(Tave), the maximum thickness (Tmax), and the minimum thickness (Tmin)were determined, and the thickness uniformity was calculated from theformula (Tmax−Tmin)/Tave.

The smaller the value, the higher the thickness uniformity. Any ofvarious commercially available film thickness meters can be used fordetermining the film thickness uniformity, such as “SME-1” manufacturedby SANKO ELECTRONIC LABORATORY CO.,LTD.

There are no particular restrictions on the thickness of the porouspolypropylene film of the present invention, and the film can range fromextremely thin to very thick, but the thickness is generally about 5 to100 μm, with about 10 to 50 μm being preferred.

The porous polypropylene film of the present invention generally has aGurley air-permeability of about 10 to 100 (sec/10 ml), particularlyabout 10 to 50 (sec/10 ml).

The porosity of the porous polypropylene film of the present inventionis preferably about 30 to 65%, particularly about 40 to 55%. Herein,“porosity” is a value determined by cutting the stretched film into asquare and measuring the length on one side (L cm), the weight (W g) andthe thickness (D cm) and calculating the value from the followingequation:porosity (%)=100−100(W/ρ)/(L ^(2×) D)wherein ρ is the density of the polypropylene unstretched web sheetprior to stretching.

The porous polypropylene film of the present invention also has goodfeeling or hand. Because of this property, the film of the presentinvention is advantageous in skin-contact applications, such asdisposable diapers, sanitary products and various packaging materials.

The porous polypropylene film of the present invention also hasproperties suited to the manufacture of a battery separator. Theestimated electrical resistance R per mil (25 μm) of film thicknesscalculated according to the following equation from the Gurleyair-permeability and the average pore size is less than 30 ohm·in/mil,and particularly 4 to 30 ohm·in/mil:R=25(4.2 t _(Gur) d)/L  (Formula 1)wherein R is the estimated electrical resistance (ohm·in/mil) of a filmin a 31 wt % KOH solution, t_(Gur) is the Gurley air-permeability(sec/10 ml) measured according to ASTM D726, d is the average pore size(μm) determined by mercury intrusion porosimetry, and L is the filmthickness (μm)).

(Formula 1) is derived from the following (Formula 2) and (Formula 3).The proportional relationship represented by (Formula 2) has been notedin the electrical resistance RmA (mohm·in²) of a film and the product(sec·μm) of the Gurley number (sec) and the average pore size (μm) (R.W. Callahan et al., The Tenth International Seminar on Primary andSecondary Battery Technology and Application, Mar. 1–4, 1993). Theestimated electrical resistance per mil (25 μm) of film thickness can becalculated from the obtained RmA and (Formula 3) (Japanese UnexaminedPatent Publication No. 2000-30683).RmA=4.2 t_(Gur)d  (Formula 2)R=25 RmA/L  (Formula 3)

The above-mentioned Gurley air-permeability (t_(Gur)) was measuredaccording to ASTM D726.

The porous polypropylene film of the present invention further has anaverage pore size of about 0.04 to 0.060 μm, particularly about 0.045 to0.055 μm, when measured by bubble point method (JIS K 3832), and isabout 0.10 to 0.50 μm, particularly about 0.20 to 0.40 μm, when measuredby mercury intrusion porosimetry. The maximum pore size in the filmthickness direction, as determined from electron microscopy (SEM) of afilm cross section, is about 0.1 to 5 μm, particularly about 0.5 to 5μm, and the maximum pore size in the direction perpendicular to thethickness direction is about 1 to 50 μm, particularly about 5 to 30 μm.

The porous polypropylene film of the present invention has a water vaporpermeability as measured according to JIS Z 0208 of generally about 3000to 6000 g/m²·24 h, particularly about 4000 to 5000 g/m²·24 h; a tensilestrength measured according to JIS K 7127 of about 50 to 100 MPa, andparticularly about 60 to 80 MPa, in both the longitudinal and transversedirections. As to leakage resistance data, the water pressure resistancemeasured according to JIS L 1092 (except that a 0.25 wt % aqueoussolution of a surfactant (sodium polyoxyethylene lauryl ether sulfate(number of moles of ethylene oxide added: 3 moles)) is used instead ofpure water) is about 200 to 400 kPa, particularly about 200 to 300 kPa.

<Polypropylene-based Resin>

The polypropylene-based resin used in the present invention is a polymerwhose main constituent component is propylene. Specific examples includepropylene homopolymers, and copolymers of propylene as major comonomerand a C₂ or C₄–C₁₂ 1-alkene, such as ethylene, butene, pentene, hexene,heptene, octene, nonene, decene, undecene or dodecene (including randomand block copolymers). The propylene content in the copolymer ispreferably at least 90 wt %, particularly 92 to 98 wt %.

Of these, a block copolymer of propylene as major comonomer and ethyleneand/or one or more 1-alkenes (having 2 or 4 to 12 carbons) is moreexcellent than a homopolymer in hand (feeling) of the obtained porouspolypropylene, and is superior to a random copolymer in terms of airpermeability and water vapor permeability, and is therefore recommended.

Other examples include blended polymers of the above-mentionedpolypropylene-based resin with a small amount of a thermoplastic resinsuch as high-density polyethylene, polybutene-1, andpoly-4-methylpentene-1. The proportion of polypropylene-based resin inthis polymer blend is preferably at least 90 wt %, particularly 92 to 98wt %.

When the polypropylene-based resin used in the present invention is apropylene-ethylene copolymer, the recommended ethylene content thereofis 3.0 to 7.0 wt %. If the ethylene content exceeds 7.0 wt %, theresulting film tends to be subject to breaking during stretching,whereas if the ethylene content is less than 3.0 wt %, uneven stretchingis likely to occur in the stretching steps, and the resulting film tendsto have very low air-permeability and deteriorated hand (feeling).

There are no particular restrictions on the melt flow rate (hereinafterreferred to as MFR; measured according to JIS K 6758-1981) of thepolypropylene-based resin, but a resin with an MFR of about 0.1 to 10g/10 minutes is usually used. A range of 1.0 to 5 g/10 minutes,preferably 2.0 to 4.0 g/10 minutes, is recommended from the standpointsof the workability and the mechanical and other properties of thestretched film. If the MFR is less than 0.1 g/10 minutes, high-speedmolding tends to be difficult and may cause decreased workability,whereas if the MFR is more than 10 g/10 minutes, the stretched filmtends to have lower mechanical properties, and breaking duringstretching is apt to occur.

<β-Crystal Nucleating Agent>

Examples of the β-crystal nucleating agent used in the present inventioninclude known potassium 12-hydroxystearate, magnesium benzoate,magnesium succinate, magnesium phthalate and other alkali or alkalineearth metal salts of carboxylic acids, sodium benzenesulfonate, sodiumnaphthalenesulfonate and other aromatic sulfonic acid compounds, di- andtriesters of di- and tribasic carboxylic acids, tetraoxaspiro compounds,imidocarboxylic acid derivatives, pigments such as phthalocyanine blueand other phthalocyanine-based pigments, quinacridone,quinacridonequinone and other quinacridone-based pigments, two-componentsystems composed of component A that is an organic dibasic acid andcomponent B that is an oxide, hydroxide or salt of an alkaline earthmetal, amide compounds represented by the following formula (1) to (3),and acid imide alkaline earth metal salts represented by the formula (4)such as a calcium salt of phthaloylglycine. Of these, amide compoundsrepresented by the formula (1) are best suited to the production ofβ-crystals because there are no problems such as coloration and soforth.R²—NHCO—R¹—CONH—R³  (1)wherein R¹ is a C₁ to C₂₄ saturated or unsaturated aliphaticdicarboxylic acid residue, a C₄ to C₂₈ saturated or unsaturatedalicyclic dicarboxylic acid residue, or a C₆ to C₂₈ aromaticdicarboxylic acid residue; R² and R³ may be the same or different, andeach represents a C₃ to C₁₈ cycloalkyl group, or a group of the formula(a), formula (b), formula (c), or formula (d):

wherein R⁴ is a hydrogen atom, a C₁ to C₁₂ straight-chain orbranched-chain alkyl group, a C₆ to C₁₀ cycloalkyl group or phenylgroup, R⁵ is a C₁ to C₁₂ straight-chain or branched-chain alkyl group,and R⁶ and R⁷ may be the same or different, and each represent a C₁ toC₄ straight-chain or branched-chain alkylene group.R⁹—CONH—R⁸—NHCO—R¹⁰  (2)wherein R⁸ is a C₁ to C₂₄ saturated or unsaturated aliphatic diamineresidue, a C₄ to C₂₈ alicyclic diamine residue, a C₄ to C₁₄ heterocyclicdiamine residue or a C₆ to C₂₈ aromatic diamine residue; R⁹ and R¹⁰ maybe the same or different, and each represent a C₃ to C₁₂ cycloalkylgroup, or a group-represented by the formula (e), formula (f), formula(g) or formula (h):

wherein R¹¹ is a hydrogen atom, a C₁ to C₁₂ straight-chain orbranched-chain alkyl group, a C₆ to C₁₀ cycloalkyl group or phenylgroup, R¹² is a C₁ to C₁₂ straight-chain or branched-chain alkyl group,a C₆ to C₁₀ cycloalkyl group or phenyl group, and R¹³ and R¹⁴ eachrepresent a C₁ to C₄ straight-chain or branched-chain alkylene group.R¹⁶—CONH—R¹⁵—CONH—R¹⁷  (3)wherein R¹⁵ is a C₁ to C₂₈ saturated or unsaturated aliphatic amino acidresidue, a C₆ to C₁₂ saturated or unsaturated alicyclic amino acidresidue or a C₆ to C₁₄ aromatic amino acid residue; R¹⁶ has the samemeaning as R² or R³ in the formula (1); and R¹⁷ has the same meaning asR⁹ or R¹⁰ in the formula (2).

wherein R¹⁸ is a hydrogen atom, a C₁ to C₁₂ straight-chain orbranched-chain alkyl group, phenyl group, benzyl group, cyclohexyl groupor carboxyl group, a is an integer of 0 to 12, and A is a dicarboxylicacid residue represented by the formula (i), formula (j), formula (k),formula (1) or formula (m):

wherein R¹⁹ is a hydrogen atom, a C₁ to C₁₂ straight-chain orbranched-chain alkyl group or a halogen atom, x is an integer of 1 to 4,y is an integer of 1 to 6, and when x and y are greater than 1, thegroups represented by R¹⁹ may be the same or different.

The amide compound represented by the formula (1) can be easily preparedby subjecting an aliphatic, alicyclic, or aromatic dicarboxylic acidrepresented by the formula (1a)HOOC—R—COOH  (1a)wherein R²⁰ has the same meaning as R¹ above and one or more alicyclicor aromatic monoamines represented by the formula (1b)R—NH₂  (1b)wherein R²¹ has the same meaning as R² or R³ above to amidation by aconventional method.

Therefore, the “dicarboxylic acid residue” represented by R¹ in theformula (1) refers to a residue (divalent group) obtained by removingtwo carboxyl groups from the following aliphatic, alicyclic, or aromaticdicarboxylic acids. R² and R³ in the formula (1) are residues obtainedby removing an amino group from the following alicyclic or aromaticamines.

Examples of aliphatic dicarboxylic acids include C₃ to C₂₆, preferablyC₃ to C₁₄, saturated or unsaturated aliphatic dicarboxylic acids.Specific examples include malonic acid, diphenylmalonic acid, succinicacid, phenylsuccinic acid, diphenylsuccinic acid, glutaric acid,3,3-dimethylglutaric acid, adipic acid, pimelic acid, suberic acid,azelaic acid, sebacic acid, 1,12-dodecanedioic acid,1,14-tetradecanedioic acid, and 1,18-octadecanedioic acid.

Examples of alicyclic dicarboxylic acids include C₆ to C₃₀, andpreferably C₈ to C₁₂, alicyclic dicarboxylic acids. Examples include1,2-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid,1,5-decalindicarboxylic acid, 2,6-decalindicarboxylic acid,4,4′-bicyclohexanedicarboxylic acid, and 1,4-cyclohexanediacetic acid.

Examples of aromatic dicarboxylic acids include C₈ to C₃₀, andpreferably C₈ to C₂₂, aromatic dicarboxylic acids, specifically includep-phenylenediacetic acid, p-phenylenediethanoic acid, phthalic acid,4-tert-butylphthalic acid, isophthalic acid, 5-tert-butylisophthalicacid, terephthalic acid, naphthalic acid, 1,4-naphthalenedicarboxylicacid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylicacid, diphenic acid, 3,3′-biphenyldicarboxylic acid,4,4′-biphenyldicarboxylic acid, 4,4′-binaphthyldicarboxylic acid,bis(3-carboxyphenyl)methane, bis(4-carboxyphenyl)methane,2,2-bis(3-carboxyphenyl)propane, 2,2-bis(4-carboxyphenyl)propane,3,3′-sulfonyldibenzoic acid, 4,4′-sulfonyldibenzoic acid,3,3′-oxydibenzoic acid, 4,4′-oxydibenzoic acid, 3,3-carbonyldibenzoicacid, 4,4′-carbonyldibenzoic acid, 3,3′-thiodibenzoic acid,4,4′-thiodibenzoic acid, 4,4′-(p-phenylenedioxy)dibenzoic acid,4,4′-isophthaloyldibenzoic acid, 4,4′-terephthaloyldibenzoic acid,dithiosalicylic acid, and other such aromatic dibasic acids.

Examples of alicyclic monoamines include C₃ to C₁₈ cycloalkylamines andcompounds represented by the formula (5)

wherein R²² has the same meaning as R⁵ above or by the formula (6)

-   wherein R²³ has the same meaning as R⁷ above. More specific examples    include cyclopropylamine, cyclobutylamine, cyclopentylamine,    cyclohexylamine, 2-methylcyclohexylamine, 3-methylcyclohexylamine,    4-methylcyclohexylamine, 2-ethylcyclohexylamine,    4-ethylcyclohexylamine, 2-propylcyclohexylamine,    2-isopropylcyclohexylamine, 4-propylcyclohexylamine,    4-isopropylcyclohexylamine, 2-tert-butylcyclohexylamine,    4-n-butylcyclohexylamine, 4-isobutylcyclohexylamine,    4sec-butylcyclohexylamine, 4-tert-butylcyclohexylamine,    4-n-amylcyclohexylamine, 4-isoamylcyclohexylamine,    4sec-amylcyclohexylamine, 4-tert-amylcyclohexylamine,    4-hexylcyclohexylamine, 4-heptylcyclohexylamine,    4octylcyclohexylamine, 4-nonylcyclohexylamine,    4-decylcyclohexylamine, 4-undecylcyclohexylamine,    4-dodecylcyclohexylamine, 4-cyclohexylcyclohexylamine,    4-phenylcyclohexylamine, cycloheptylamine, cyclododecylamine,    cyclohexylmethylamine, α-cyclohexylethylamine,    β-cyclohexylethylamine, α-cyclohexylpropylamine,    β-cyclohexylpropylamine and γ-cyclohexylpropylamine.

Examples of aromatic monoamines include compounds represented by theformula (7)

-   wherein R²⁴ has the same meaning as R⁴ above or by the formula (8)

-   wherein R²⁵ has the same meaning as R⁶ above. More specific examples    include aniline, o-toluidine, m-toluidine, p-toluidine,    o-ethylaniline, p-ethylaniline, o-propylaniline, m-propylaniline,    p-propylaniline, o-cumidine, m-cumidine, p-cumidine,    o-tert-butylaniline, p-n-butylaniline, p-isobutylaniline,    p-sec-butylaniline, p-tert-butylaniline, p-n-amylaniline,    p-isoamylaniline, p-sec-amylaniline, p-tert-amylaniline,    p-hexylaniline, p-heptylaniline, p-octylaniline, p-nonylaniline,    p-decylaniline, p-undecylaniline, p-dodecylaniline,    p-cyclohexylaniline, o-aminodiphenyl, m-aminodiphenyl,    p-aminodiphenyl, benzylamine, α-phenylethylamine,    β-phenylethylamine, α-phenylpropylamine, β-phenylpropylamine and    γ-phenylpropylamine.

Of the amide compounds represented by the formula (1), examples ofparticularly favorable compounds include N,N′-diphenylhexanediamide,N,N′-dicyclohexylterephthaldiamide, andN,N′-dicyclohexyl-2,6-naphthalenedicarboxamide, etc.

The amide compound represented by the formula (2) can be easily preparedby subjecting an aliphatic, alicyclic or aromatic diamine represented bythe formula (2a) and one or more alicyclic or aromatic monocarboxylicacid represented by the formula (2b) to amidation by a conventionalmethod:H₂N—R²⁶—NH₂  (2a)wherein R²⁶ has the same meaning as R⁸ aboveR²⁷—COOH  (2b)wherein R²⁷ has the same meaning as R⁹ or R¹⁰ above.

Therefore, the “diamine residue” represented by R⁸ in the formula (2)refers to a residue (divalent group) obtained by removing two aminogroups from the following aliphatic, alicyclic or aromatic diamines. R⁹and R¹⁰ in the formula (2) are residues obtained by removing a carboxylgroup from the following alicyclic or aromatic monocarboxylic acids.

Examples of aliphatic diamines include C₁ to C₂₄, preferably C₁ to C₁₂,aliphatic diamines. Specific examples include 1,2-diaminoethane,1,2-diaminopropane, 1,3-diaminopropane, 1,4-diaminobutane,1,3-diaminopentane, 1,5-diaminopentane, 1,6-diaminohexane,1,8-diaminooctane, 1,10-diaminodecane and 1,11-diaminoundecane.

Examples of alicyclic diamines include C₄ to C₂₈, preferably C₆ to C₁₅,alicyclic diamines. Specific examples include 1,2-diaminocyclohexane,1,4-diaminocyclohexane, 4,4′-diaminodicyclohexyl,4,4′-diamino-3,3′-dimethyldicyclohexyl, 4,4′-diaminodicyclohexylmethane,4,4′-diamino-3,3′-dimethyldicyclohexylmethane,1,3-bis(aminomethyl)cyclohexane, and 1,4-bis(aminomethyl)cyclohexane, aswell as isophoronediamine, menthenediamine, and so forth.

Examples of heterocyclic diamines include five- and six-membered C₄ toC₁₄ heterocyclic diamines containing one or two nitrogen atoms or sulfuratoms in their ring structure. Specific examples include2,3-diaminopyridine, 2,6-diaminopyridine, 3,4-diaminopyridine,o-tolidinesulfone and the like.

Examples of aromatic diamines include C₆ to C₂₈, preferably C₆ to C₁₅,aromatic diamines. Specific examples include o-phenylenediamine,m-phenylenediamine, p-phenylenediamine, 2,3-diaminotoluene,2,4-diaminotoluene, 2,6-diaminotoluene, 3,4-diaminotoluene,4,6-dimethyl-m-phenylenediamine, 2,5-dimethyl-p-phenylenediamine,4,5-dimethyl-o-phenylenediamine, o-xylylenediamine, m-xylylenediamine,p-xylylenediamine, 2,4-diaminomesitylene, 1,5-diaminonaphthalene,1,8-diaminonaphthalene, 2,3-diaminonaphthalene, 2,7-diaminonaphthalene,9,10-diaminophenanthrene, 3,3′,5,5′-tetramethylbenzidine,3,3′-dimethyl-4,4′-diaminobiphenyl, 3,3′-dimethoxy-4,4′-diaminobiphenyl,4,4′-diaminodiphenylmethane, 3,3′-diaminodiphenylmethane,3,4′-diaminodiphenylmethane, 4,4′-methylenebis(o-toluidine),4,4′-methylenebis(2,6-xylidine), 4,4′-methylenebis(2,6-diethylaniline),4,4′-diamino-1,2-diphenylethane, 4,4′-diamino-2,2′-dimethylbibenzyl,4,4′-diaminostilbene, 3,4′-diamino-2,2-diphenylpropane,4,4′-2,2-diphenylpropane, 4,4′-diaminodiphenyl ether,3,4′-diaminodiphenyl ether, 4,4′-thiodianiline, 2,2′-dithiodianiline,4,4′-dithiodianiline, 3,3′-diaminodiphenylsulfone,4,4′-diaminodiphenylsulfone, 3,3′-diaminobenzophenone,4,4′-diaminobenzophenone, 4,4′-diaminobenzanilide, 2,7-diaminofluorene,3,7-diamino-2-methoxyfluorene, bis-p-aminophenylaniline,1,3-bis(4-aminophenylpropyl)benzene,1,4-bis(4-aminophenylpropyl)benzene, 1,3-bis(4-aminophenoxy)benzene,4,4′-bis(4-aminophenoxy)biphenyl, bis[4-(4-aminophenoxy)phenyl] ether,bis[4-(4-aminophenoxy)phenyl]sulfone, 9,9-bis(4-aminophenyl)fluorene,and the like.

Examples of alicyclic monocarboxylic acids include C₄ to C₁₃cycloalkanecarboxylic acids, C₄ to C₁₃ cycloalkenecarboxylic acids, andcompounds represented by the formulas (9) and (10). Specific examplesinclude cyclopropanecarboxylic acid, cyclobutanecarboxylic acid,cyclopentanecarboxylic acid, 1-methylcyclopentanecarboxylic acid,2-methylcyclopentanecarboxylic acid, 3-methylcyclopentanecarboxylicacid, 1-phenylcyclopentanecarboxylic acid, cyclopentenecarboxylic acid,cyclohexanecarboxylic acid, 1-methylcyclohexanecarboxylic acid,2-methylcyclohexanecarboxylic acid, 3-methylcyclohexanecarboxylic acid,4-methylcyclohexanecarboxylic acid, 4-propylcyclohexanecarboxylic acid,4-butylcyclohexanecarboxylic acid, 4-pentylcyclohexanecarboxylic acid,4-hexylcyclohexanecarboxylic acid, 4-phenylcyclohexanecarboxylic acid,1-phenylcyclohexanecarboxylic acid, cyclohexenecarboxylic acid,4-butylcyclohexenecarboxylic acid, cycloheptanecarboxylic acid,1-cycloheptenecarboxylic acid, 1-methylcycloheptanecarboxylic acid,4-methylcycloheptanecarboxylic acid, and cyclohexylacetic acid.

wherein R²⁸ has the same meaning as R¹² above.

wherein R²⁹ has the same meaning as R¹⁴ above.

Examples of aromatic monocarboxylic acids include the compoundsrepresented by the formulas (11) and (12). Specific examples includebenzoic acid, o-methylbenzoic acid, m-methylbenzoic acid,p-methylbenzoic acid, p-ethylbenzoic acid, p-propylbenzoic acid,p-butylbenzoic acid, p-tert-butylbenzoic acid, p-pentylbenzoic acid,p-hexylbenzoic acid, o-phenylbenzoic acid, p-phenylbenzoic acid,p-cyclohexylbenzoic acid, phenylacetic acid, phenylpropionic acid,phenylbutyric acid and the like.

wherein R³⁰ has the same meaning as R¹¹ above.

wherein R³¹ has the same meaning as R¹³ above.

Of the amide compounds represented by the formula (2), examples ofparticularly favorable compounds includeN,N′-dicyclohexanecarbonyl-p-phenylenediamine,N,N′-dibenzoyl-1,5-diaminonaphthalene,N,N′-dibenzoyl1,4-diaminocyclohexane, andN,N′-dicyclohexanecarbonyl1,4-diaminocyclohexane.

The amide compounds represented by the formula (3) can be easilyprepared by subjecting an aliphatic, alicyclic or aromatic amino acidrepresented by the formula (3a) and a specific monocarboxylic acid andmonoamine to amidation by a conventional method.HOOC—R³²—NH₂  (3a)wherein R³² has the same meaning as R¹⁵ above.

Therefore, the “amino acid residue” represented by R¹⁵ in the formula(3) refers to a residue (divalent group) obtained by removing one aminogroup and one carboxyl group from the following aliphatic, alicyclic oraromatic amino acids.

Examples of aliphatic amino acids include C₂ to C₂₉, preferably C₂ toC₁₃, saturated or unsaturated aliphatic amino acids. Specific examplesinclude aminoacetic acid, α-aminopropionic acid, β-aminopropionic acid,α-aminoacrylic acid, α-aminobutyric acid, β-aminobutyric acid,γ-aminobutyric acid, α-amino-α-methylbutyric acid,γ-amino-α-methylenebutyric acid, α-aminoisobutyric acid,β-aminoisobutyric acid, α-amino-n-valeric acid, δ-amino-valeric acid,β-aminocrotonic acid, α-amino-β-methylvaleric acid, α-aminoisovalericacid, 2-amino-4-pentenoic acid, (α-amino-n-caproic acid, 6-aminocaproicacid, α-aminoisocaproic acid, 7-aminopentanoic acid, α-amino-n-caprylicacid, 8-aminocaprylic acid, 9-aminononanoic acid, 11-aminoundecanoicacid, 12-aminododecanoic acid and the like.

Examples of alicyclic amino acids include C₇ to C₁₃, saturated orunsaturated alicyclic amino acids. Specific examples include1-aminocyclohexanecarboxylic acid, 2-aminocyclohexanecarboxylic acid,3-aminocyclohexanecarboxylic acid, 4-cyclohexanecarboxylic acid,p-aminomethylcyclohexane-carboxylic acid, and2-amino-2-norbornanecarboxylic acid.

Examples of aromatic amino acids include C₇ to C₁₅ aromatic amino acids.Specific examples include α-aminophenylacetic acid,α-amino-β-phenylpropionic acid, 3-amino-3-phenylpropionic acid,α-aminocinnamic acid, 2-amino-4-phenylbutyric acid,4-amino-3-phenylbutyric acid, anthranilic acid, m-aminobenzoic acid,p-aminobenzoic acid, 2-amino-4-methylbenzoic acid,2-amino-6-methylbenzoic acid, 3-amino-4-methylbenzoic acid,2-amino-3-methylbenzoic acid, 2-amino-5-methylbenzoic acid,4-amino-2-methylbenzoic acid, 4-amino-3-methylbenzoic acid,2-amino-3-methoxybenzoic acid, 3-amino-4-methoxybenzoic acid,4-amino-2-methoxybenzoic acid, 4-amino-3-methoxybenzoic acid,2-amino-4,5-dimethoxybenzoic acid, o-aminophenylacetic acid,m-aminophenylacetic acid, p-aminophenylacetic acid,4-(4-aminophenyl)butyric acid, 4-aminomethylbenzoic acid,4-aminomethylphenylacetic acid, o-aminocinnamic acid, m-aminocinnamicacid, p-aminocinnamic acid, p-aminohippuric acid, 2-amino-1-naphthoicacid, 3-amino1-naphthoic acid, 4-amino-1-naphthoic acid,5-amino-1-naphthoic acid, 6-amino-1-naphthoic acid, 7-amino-1-naphthoicacid, 8-amino-1-naphthoic acid, 1-amino-2-naphthoic acid,3-amino-2-naphthoic acid, 4-amino-2-naphthoic acid, 5-amino-2-naphthoicacid, 6-amino-2-naphthoic acid, 7-amino-2-naphthoic acid,6-amino-2-naphthoic acid, 7-amino-2-naphthoic acid, and8-amino-²-naphthoic acid.

The monoamine that is the raw material of the amide compound representedby the formula (3) is the same as the monoamine that is the raw materialof the amide compound represented by the formula (1), and similarly, themonocarboxylic acid is the same as the monocarboxylic acid that is theraw material of the amide compound represented by the formula (2).

Of the amide compounds represented by the formula (3), examples of moreeffective compounds includeN-cyclohexyl-4-(N-cyclohexanecarbonyl-amino)benzamide andN-phenyl-5-(N-benzoylamino)pentaneamide.

The “dicarboxylic acid residue” represented by A in the formula (4)refers to a group (divalent group) obtained by removing two carboxylgroups from an aromatic or alicyclic dicarboxylic acid.

Examples of alkaline earth metals include magnesium, calcium, andbarium, of which calcium is particularly favorable.

Of the acid imide compounds represented by the formula (4), examples ofmore effective compounds include calcium salts of phthaloylglycine,hexahydrophthaloylglycine, naphthoylglycine, N-phthaloylalanine,N-4-methylphthaloylglycine, and so on, with calcium salt ofphthaloylglycine being particularly favorable.

The acid imide compounds represented by the formula (4) are knowncompounds, and can be easily prepared by subjecting a specific alicyclicor aromatic dicarboxylic anhydride and a specific amino acid toimidation by a conventional method, such as the method disclosed inEP0887375A1.

It is recommended that the β-crystal nucleating agent used in thepresent invention, and particularly the amide compound, have a maximumparticle diameter of not more than 20 μm, preferably not more than 10μm, more preferably 5 μm or less. A maximum particle diameter exceeding20 μm may lead to breakage during stretching.

It is recommended that the amount of the β-crystal nucleating agent ofthe present invention to be used be 0.0001 to 5 weight parts, preferably0.001 to 1 weight part, per 100 weight parts of the polypropylene-basedresin. If the amount is less than 0.0001 weight part, β-crystals tendnot to be produced in a sufficient quantity, whereas if the amountexceeds 5 weight parts, marked improvement in effect is not observed,and furthermore breakage may be caused in the stretching step.

Conventional polyolefin modifiers can be added to thepolypropylene-based resin according to the present invention as dictatedby the intended use and application, to the extent that the effect ofthe present invention is not impaired.

Examples of such polyolefin modifiers include various additivesdiscussed in “Digest of Positive List of Additives” (January, 1995)edited by Japan Hygienic Olefin And Styrene Plastics Association. Morespecifically, examples include stabilizers (such as metal compounds,epoxy compounds, nitrogen compounds, phosphorus compounds, and sulfurcompounds), UV absorbers (such as benzophenone compounds andbenzotriazole compounds), antioxidants (such as phenol compounds,phosphorous ester compounds, and sulfur compounds), surfactants,lubricants (such as paraffin, wax, and other aliphatic hydrocarbons, C₈to C₂₂ higher fatty acids, C₈ to C₂₂ higher fatty acid metal (Al, Ca,Mg, Zn) salts, C₈ to C₁₈ fatty acids, C₈ to C₂₂ aliphatic alcohols,polyglycols, esters of C₄ to C₂₂ higher fatty acids and C₄ to C₁₈aliphatic monohydric alcohols, C₈ to C₂₂ higher fatty acid amides,silicone oils, and rosin derivatives), fillers (such as talc,hydrotalcite, mica, zeolite, perlite, diatomaceous earth, calciumcarbonate, and glass fiber), foaming agents, foaming auxiliaries,polymer additives, plasticizers, crosslinking agents, crosslinkingauxiliaries, antistatic agents, neutralizers, anti-blocking agents,anti-fogging agents, polymer alloy components (such as blocked SBR,random SBR, hydrogenated products thereof, and like rubbers, andpolystyrenes), flame retardants, dispersants, organic and inorganicpigments and dyes, and working auxiliaries.

Process for Producing the Porous Polypropylene Film of the PresentInvention

The process for producing the successively biaxially stretchedpolypropylene porous film of the present invention is a process forproducing a successively biaxially stretched, β-crystal nucleatingagent-containing polypropylene porous film by a sequential biaxiallystretching step in which an unstretched polypropylene web sheetcontaining a β-crystal nucleating agent is longitudinally stretched andthen transversely stretched, characterized in that the degree oforientation of β-crystals calculated from a pole figure of the crystallattice (300) plane of the β-crystals determined by X-ray diffraction ofthe sheet obtained after longitudinal stretching is adjusted to lessthan 0.30 by performing the following method (I) and/or method (II).

Method (I): As a β-crystal nucleating agent, needle crystals of theamide compound discussed in item 6 above is used, and the resintemperature during extrusion from a T-die is set to be higher than themelting point of the polypropylene and lower than the temperature atwhich the amide compound dissolves in the polypropylene-based resinmelt, and the melt of the polypropylene-based resin composition isextruded from the T-die in a state in which the needle crystals of theamide compound are present.

Method (II): The neck-in ratio during longitudinal stretching isadjusted to at least 25% and not more than 55%.

The recommended porous polypropylene manufacturing conditions will nowbe described in detail by going through the manufacturing steps.

<Polypropylene-based Resin Composition>

The polypropylene-based resin composition according to method (I)contains needle crystals of an amide compound which is a β-crystalnucleating agent. This polypropylene-based resin composition is preparedas follows.

The polypropylene-based resin and the amide compound are mixed, forexample, in a Henschel mixer and the resulting mixture is melt-kneadedin a single screw or double screw extruder or the like at a temperaturewhich is not lower than the temperature at which the amide compounddissolves in the polypropylene-based resin melt but not higher than 280°C., whereby the amide compound is homogeneously dissolved in thepolypropylene-based resin melt. Then, this product is cooled and cutinto resin pellets. The resin pellets thus obtained contain needlecrystals of the amide compound.

The above-mentioned dissolution temperature varies with the type ofpolypropylene-based resin and with the type of amide compound and theaddition level thereof. As the amide compound content is increased, thedissolution temperature rises. For instance, whenN,N′-dicyclohexyl-2,6-naphthalenecarboxamide is used as the β-crystalnucleating agent of the present invention and the content thereof isincreased to 0.04, 0.05, 0.06, 0.1 and 0.2 weight part, the dissolutiontemperature rises to about 235° C., about 240° C., about 245° C., about260° C., and about 280° C. respectively.

Therefore, when the amide compound content is 0.05 weight part, the meltkneading must be performed at a temperature of at least 240° C.Degradation of the resin becomes pronounced if the kneading temperatureis over 280° C., and this can lead to coloration of the resin or tobreakage during stretching.

It is preferable that the β-crystal nucleating agent used in method (I)are:

(1) at least one member selected from the group consisting ofN,N′-diphenylhexanediamide, N,N′-dicyclohexylterephthalamide andN,N′-dicyclohexyl-2, 6-naphthalenedicarboxamide,

(2) at least one member selected from the group consisting ofN,N′-dicyclohexanecarbonyl-p-phenylenediamine,N,N′-dibenzoyl-1,5-diaminonaphthalene,N,N′-dibenzoyl-1,4-diaminocyclohexane andN,N′-dicyclohexanecarbonyl-1,4-diaminocyclohexane,

(3) at least one member selected from the group consisting ofN-cyclohexyl-4-(N-cyclohexanecarbonyl-amino)benzamide andN-phenyl-5-(N-benzoylamino)pentaneamide, or

(4) a mixture of two or more members of the above amide compounds of (1)to (3).

The polypropylene-based resin composition according to method (II)may-be prepared as follows. A polypropylene-based resin and theβ-crystal nucleating agent represented by the formulas (1) to (4)according to the present invention are mixed in a Henschel mixer andthen melt-kneaded at 200 to 280° C., regardless of the meltingtemperature of the amide compound, and then this melt is cooled and cutinto resin pellets. The pellets thus obtained contain columnar crystalsor needle crystals of a β-crystal nucleating agent (e.g., the amidecompound of the above-mentioned formulas (1) to (3)).

The polyolefin modifiers which may be used as needed in the presentinvention may be compounded during the preparation of thepolypropylene-based resin, or they may be added by mixing them withseparately prepared resin.

<Unstretched Web Sheet>

The unstretched polypropylene web sheet according to method (I) isobtained by preparing a polypropylene-based resin composition containingneedle crystals of a β-crystal nucleating agent, i.e., the amidecompound set forth in item 6 above, extruding the composition from aT-die at a temperature which is not lower than the melting point of thepolypropylene-based resin and lower than the temperature at which theamide compound dissolves in the polypropylene-based resin melt, and thencooling the molten sheet thus obtained. Under these extrusiontemperature conditions, the β-crystals of the amide compound areextruded without dissolving in the polypropylene-based resin melt, withthe result that an unstretched web sheet is obtained in which β-crystallamella layers are highly oriented.

The unstretched polypropylene web sheet according to method (II) isobtained by preparing a polypropylene-based resin composition containingthe β-crystal nucleating agent according to the present invention,extruding this composition from a T-die at a resin temperature of about200 to 280° C., preferably about 230 to 250° C., regardless of thedissolution temperature of the β-crystal nucleating agent represented bythe formulas (1) to (4) above, and then cooling and crystallizing themolten sheet thus obtained.

If the resin temperature is excessively lower than 200° C., the resin islikely to partly remain unmelted, possibly leading to breakage duringstretching, whereas if the resin temperature is higher than 280° C.,this can lead to degradation of the resin, breakage in the stretchingstep, coloration of the resin, etc. However, the molten state anddegradation situation of the resin vary considerably with the type ofthe resin and the stabilizer used, so that the resin temperature may notnecessarily need to be within the temperature range given above.

In both method (I) and method (II), the β-crystals ofpolypropylene-based resin are produced when the extruded β-crystalnucleating agent-containing polypropylene-based resin melt is cooled andcrystallized, and the crystallization temperature for efficientlyproducing these β-crystals, that is, the chill roll temperature, is 110to 130° C., preferably 115 to 125° C., more preferably 120° C. Thecrystallization holding time, that is, the contact time between thechill roll and the sheet is 10 to 60 seconds, preferably 12 to 30seconds, more preferably 15 to 20 seconds.

The chill roll temperature of lower than 110° C. increases formation ofunstable β-crystals which contribute less to pore formation, henceundesirable. The chill roll temperature exceeding 130° C. is alsoundesirable because crystallization will take a long time andproductivity will be adversely affected.

The chill roll contact time of less than 10 seconds is not preferable,because crystallization is incomplete and unstable β-crystals increase.The production of β-crystals is usually complete when the contact timeis 60 seconds.

The β-crystal content in the obtained unstretched web sheet can beselected from a wide range, but it is generally preferable that theβ-crystal content is 60 to 90%, particularly 70 to 80%. “β-crystalcontent” as used herein is determined by cutting an unstretchedpolypropylene web sheet to a suitable size, subjecting this sample todifferential scanning calorimetry (DSC) in a nitrogen atmosphere and ata heating rate of 20° C./min, and then calculating the β-crystal contentaccording to the following equation using the heat of fusion of a andβ-crystals obtained from this DSC thermogram.β-crystal content (%)=100×Hβ/(Hβ+Hα)where Hβ is the heat of fusion (units: J/g) of the β-crystals, and Hα isthe heat of fusion (units: J/g) of the α-crystals.

There are no particular restrictions on the K value of the unstretchedweb sheet, but it is generally preferable that the K value be about 0.98to 0.70, particularly 0.96 to 0.80.

The width of the unstretched web sheet is suitably selected according tothe size of the finished product and so forth. It is generallypreferable that the width is about 100 to 1000 mm, particularly 200 to600 mm, but is not limited to this range.

The thickness of the unstretched web sheet is also suitably selectedaccording to the size of the finished product and so forth. It isgenerally preferable that the thickness is about 50 to 1000 μm,particularly 100 to 500 μm, but is not limited to this range.

<Longitudinal Stretching>

The above-mentioned unstretched web sheet is then continuously guided tolongitudinal stretching rolls, and longitudinally stretched by utilizingthe rotational speed difference between the rolls. This longitudinalstretching may also be divided into several steps using a plurality ofstretching rolls. The stretching temperature, that is, the roll surfacetemperature, is 70 to 140° C., preferably 90 to 120° C., and the totallongitudinal stretch ratio is preferably 3 to 5 times. If the stretchingtemperature is lower than 70° C., uniform stretching is difficult, andif the stretching temperature exceeds 140° C., the obtained film tendsto have lower air-permeability.

When the unstretched web sheet is stretched longitudinally, theunstretched web sheet shrinks in its width direction (transversely), andthe sheet width decreases. This shrinkage is what is meant by theneck-in ratio according to method (II) of the present invention.Specifically, the neck-in ratio is a value determined as follows.Neck-in ratio (%)=100×(W−W ₁)/Wwherein W is the width of the unstretched web sheet, and W₁ is the widthof the longitudinally stretched sheet.

The neck-in ratio according to method (II) can be controlled by varyingthe width of the unstretched web sheet and/or the distance between thestretching rolls during this longitudinal stretching. The neck-in ratiois adjusted to between 25 and 55% or higher, preferably between 35 and55% or higher, more preferably 40 to 55%. By controlling this neck-inratio, the degree of β-crystal orientation <cos²θ_(TD)> of the sheetafter longitudinal stretching becomes less than 0.30, preferably lessthan 0.28, more preferably less than 0.27.

Specifically, by adjusting the neck-in ratio to 25 to 55% or higher, thedegree of β-crystal orientation <cos²θ_(TD)> is adjusted to less than0.30, and preferably by adjusting the neck-in ratio to 35 to 55% orhigher, the degree of β-crystal orientation <cos²θ_(TD)> is adjusted toless than 0.28, and more preferably by adjusting the neck-in ratio to 40to 55% or higher, the degree of crystal orientation <cos²θ_(TD)> isadjusted to less than 0.27.

If the neck-in ratio according to the present invention is less than25%, the degree of orientation of β-crystals is low and the effect ofpromoting pore formation is low. The pore formation promoting effecttends to be saturated at a neck-in ratio of 55%. The neck-in ratio canbe easily controlled by varying the ratio (W/L) between the unstretchedweb sheet width (W) and the distance between the longitudinal stretchingrolls (L). For instance, as discussed in “Kobunshi Kako One Point (Hintsfor Macromolecular Processing) Vol. 2, “Film wo tsukuru (MakingFilms)””, published on Oct. 5, 1988 by Kyoritsu Shuppan, therelationship of (Formula I) exists between the neck-in ratio and W/L.Neck-in ratio (%)=a(W/L)+b  (Formula I)(The constants a and b vary with the polypropylene-based resin servingas the raw material, the K value of the unstretched web sheet, and thelongitudinal stretching temperature and stretch ratio.)

For example, when an unstretched web sheet of β-crystal polypropylenewith a melt flow rate of 2.7 g/10 minutes and with a K value of 0.96 waslongitudinally stretched to a ratio of 4 times, the relationship of(Formula II) was obtained.Neck-in ratio (%)=−16(W/L)+56  (Formula II)

Thus, the W/L ratio for achieving a neck-in ratio of 25%, 35% and 45% is1.9, 1.3, and 0.7, respectively, and the greater the distance L betweenrolls and/or the narrower the unstretched web sheet width W, the higherthe neck-in ratio.

The W/L ratio can be varied by changing the distance between the T-dieand the chill roll (air gap) so as to vary the unstretched web sheetwidth, and/or by changing the distance between the longitudinalstretching rolls.

The longitudinal stretching roll distance varies with the roll diameter,unstretched web sheet width, and other factors, but is generally about100 to 2000 mm, and preferably about 200 to 1000 mm. It may, however, beoutside this range.

The degree of orientation of β-crystals increases with an increase inthe neck-in ratio. The degree of orientation of β-crystals according tothe present invention is the <cos²θ_(TD)> that is the mean square valueof cosθ_(TD) calculated from the pole figure data of the crystal lattice(300) plane in β-crystals obtained by X-ray diffraction.

Herein, θ_(TD) is the angle formed between the TD axis (when the widthdirection (TD) of the film is used as the main reference axis) and theaveraged reciprocal lattice vector of the (300) plane determined from anormalized orientation distribution function.

When the β-crystals are not oriented, <cos²θ_(TD)>=1/3, and when theβ-crystals are completely oriented, <cos²θ_(TD)>=0 (Kobunshi Jikkengaku,Vol. 17, Solid Structures of Macromolecules II, Kyoritsu Shuppan(1985)). The β-crystal orientation <cos²θ_(TD)> according to the presentinvention is less than 0.30, preferably less than 0.28, more preferablyless than 0.27. If the degree of β-crystal orientation is 0.30 orhigher, the orientation of β-crystals is low and effect of promotingpore formation is low. There are no particular restrictions on the lowerlimit to the degree of β-crystal orientation <cos²θ_(TD)>, but about 0.1is generally adequate. Of course, a lower value may also be employed.

As to the unstretched web sheet obtained in method (I), on the otherhand, it is not necessarily required to increase the neck-in ratio byusing method (II) in the subsequent longitudinal stretching step, andeven at the neck-in ratio which is ordinarily employed (at least 5% andless than 25%), the β-crystal lamella layers becomes oriented in thesheet after the longitudinal stretching in the same manner as whenmethod (II) is employed, with the result that the degree of β-crystalorientation <cos²θ_(TD)> of the longitudinally stretched sheet becomesless than 0.30, preferably less than 0.28, more preferably less than0.27, thereby producing a sufficient pore formation promoting effect.

By combining method (I) and method (II), however, it is possible tofurther increase the degree of orientation of the β-crystal lamellalayers, and this allows pore formation to be promoted to the maximum.

<Annealing>

If desired, the longitudinally stretched sheet can be annealed underspecific conditions after the longitudinal stretching and prior totransverse stretching. This further promotes pore formation in thesubsequent transverse stretching, improves the porous film propertiessuch as its porosity and air-permeability, and also further improvesthickness uniformity.

When the annealing treatment according to the present invention isperformed, the longitudinally stretched sheet is annealed afterlongitudinal stretching and prior to transverse stretching, at 130 to160° C. for 1 to 300 seconds and at a longitudinal stretch ratio of 0 to30%, preferably at 140 to 150° C. for 1 to 60 seconds and at alongitudinal stretch ratio of 0 to 20%, more preferably at 145 to 150°C. for 1 to 10 seconds and at a longitudinal stretch ratio of 0 to 10%.

Herein, the longitudinal stretch ratio is the value calculated from thefollowing equation.Longitudinal stretch ratio(%)=[(L₂−L₁)/L₁]×100wherein L₁ is the length of the longitudinally stretched sheet prior toannealing, and L₂ is the length of the longitudinally stretched sheetafter annealing.

By this annealing, β-crystals remaining after longitudinal stretchingundergo a crystal transition to α-crystals. If the annealing temperatureis lower than 130° C., the crystal transition from β-crystals toα-crystals becomes inadequate. It is not preferable to carry out theannealing at a temperature higher than 160° C., because the a-crystalsmelt and the orientation is disturbed. Also, it is undesirable if theannealing time is shorter than 1 second, because the crystal transitionfrom β-crystals to α-crystals becomes inadequate. The crystal transitionis almost saturated when the annealing time is approximately 300seconds.

It is also favorable to perform some very slight stretching in thelongitudinal direction during this annealing, but care should be takento prevent shrinkage from occurring. Such shrinkage, if any, woulddisturb the oriented state of the polypropylene crystals, impairing poreformation, giving a film having decreased porosity and air-permeability.Stretching beyond a longitudinal stretch ratio of 30% is undesirable,because there is no further improvement and breakage may occur.

This annealing can be accomplished by using a temperature maintainingequipment which satisfies the annealing conditions of the presentinvention, such as a pre-heating zone before stretching, inside thetransverse stretching apparatus, a hot air heater, an infrared (IR)heater, a heating roll, an oven, a hot bath, or the like, which may beused singly or in combination. It is particularly favorable to use aheating roll, because it is directly contacted with the sheet, wherebythe crystal transition from β-crystals to α-crystals smoothly proceedsand is complete in a short time of about 1 to 10 seconds.

More specifically, methods for performing this annealing treatmentinclude 1) a method in which a heating apparatus such as a hot airheater, an infrared heater, a heating roll, an oven, or a hot bath isinstalled, either singly or in combination, between the longitudinalstretching apparatus and the transverse stretching apparatus, so as toprovide a heating zone that satisfies the annealing conditions of thepresent invention, 2) a method in which the annealing is performed usinga heating roll at the final end of the longitudinal stretchingapparatus, and 3) a method in which the annealing is performed using apreheating zone at the very front of the transverse stretchingapparatus. These methods 1), 2) and 3) may be used singly or incombination.

Also, in the annealing of the longitudinally stretched sheet, it ispreferable not to allow the longitudinally stretched sheet to shrinklongitudinally, and to this end it is good to employ a means such as aroll for preventing sheet slipping (pinch roll) before and after theannealing zone so that there will be no change in the length of thelongitudinally stretched sheet in the longitudinal direction.

<Transverse Stretching>

Next, the longitudinally stretched sheet or the annealed longitudinallystretched sheet is guided to the transverse stretching apparatus, whereit is transversely stretched at a stretching temperature of 120 to 155°C., preferably 140 to 150° C., at a stretch ratio of 4 to 10 times,preferably 6 to 8 times, and at a transverse stretching strain rate of10 to 300%/sec, preferably 20 to 200%/sec, more preferably 40 to150%/sec.

Herein, the transverse stretching strain rate is the rate determined asthe ratio Vt/Dt (or 100 Vt %/Dt) of the transverse stretching rate Vt tothe width of the longitudinally stretched sheet Dt.

If the stretching temperature is under 120° C., the sheet breakage islikely to occur in the stretching step, and if the stretchingtemperature exceeds 155° C., air-permeability decreases. Productivity ispoor if the stretch ratio is less than 4 times, and the stretch ratioexceeding 10 times can lead to breakage in the transverse stretchingstep.

With a conventional manufacturing method, the transverse stretchingstrain rate greatly affects pore formation and breakage duringtransverse stretching. For example, if the strain rate is increased to100%/sec, there is a marked drop in air-permeability, and the likelihoodof breakage also increases, so that a strain rate of 17%/sec or less isrecommended. When the manufacturing method of the present invention isapplied, however, it is possible to manufacture a porous film withsufficient air-permeability and with no breakage even at a high strainrate of 100 to 300%/sec.

The porous film thus obtained is characterized in that it exhibits theabove-mentioned pore structures (a) and (b) when a film cross section isobserved by electron microscope.

EXAMPLES

Examples and comparative examples will now be given to describe thepresent invention in more detail. The temperature at which the amidecompound dissolved in the polypropylene-based resin melt, the K value ofthe unstretched web sheet, the β-crystal content, the degree ofβ-crystal orientation of the longitudinally stretched sheet, how manytimes breakage occurred during transverse stretching, and the porosity,average pore size, maximum pore size, Gurley air-permeability, estimatedelectrical resistance, water vapor permeability, leakage resistance,tensile strength, hand (feeling), and thickness uniformity of the porousfilm were determined by the methods given below.

Dissolution Temperature

In Examples and Comparative Examples, the temperature at which thecrystals of amide compound dissolved in the polypropylene-based resinmelt was determined by observing resin pellets to be extruded from aT-die, at a heating rate of 10° C./minute using an optical microscopeequipped with a temperature elevation apparatus. Whether or not theamide compound completely dissolved in the polypropylene-based resinmelt was checked by direct visual observation of the molten resin andmolten sheet during melt-mixing and during T-die extrusion of the resin.When the dissolution is complete, the molten resin is transparent,whereas the resin is white and translucent or turbid when thedissolution is not complete.

K Value

An unstretched web sheet was subjected to X-ray diffraction, and the Kvalue was determined from the following equation.K value=H(β₁)/[H(β₁)+H(α₁)+H(α₂)+H(α₃)]

-   -   H(β₁): diffraction intensity (height) of β-crystal (300) plane    -   H(α₁): diffraction intensity (height) of α-crystal (110) plane    -   H(α₂): diffraction intensity (height) of α-crystal (040) plane    -   H(α₃): diffraction intensity (height) of α-crystal (130) plane        β-Crystal Content

An unstretched polypropylene web sheet was cut to a suitable size, andthis sample was subjected to differential scanning calorimetry (DSC) ina nitrogen atmosphere and at a heating rate of 20° C./min. The β-crystalcontent was determined by the following equation using the heat offusion of α-crystals and β-crystals obtained from this DSC thermogram.β-crystal content (%)=100×Hβ/(Hβ+Hα)wherein Hβ is the heat of fusion (units: J/g) of the β-crystals, and Hαis the heat of fusion (units: J/g) of the α-crystals.Number of Breakage During Transverse Stretching

This is the number of times the porous film breakage occurred duringcontinuous manufacture over a period of 1 hour. Since even a singlebreakage markedly reduces productivity, there should be no breaks duringoperation for 1 hour.

Degree of β-Crystal Orientation of Longitudinally Stretched Sheet

The degree of orientation of the β-crystal lamella layers was determinedby calculating the mean square value <cos²θ_(TD)> of costs from the polefigure data of the crystal lattice (300) plane of β-crystals determinedby X-ray diffraction using a polar sample stage, and this was termed thedegree of orientation of β-crystals. θ_(TD) here is the angle formedbetween the TD axis (when the width direction (TD) of the film is usedas the main reference axis) and the averaged reciprocal lattice vectorfor the (300) plane determined from a normalized orientationdistribution function. When the β-crystals are not oriented,<cos²θ_(TD)>=1/3, and when the β-crystals are completely oriented,<cos²θ_(TD)>=0 (see Kobunshi Jikkengaku, Vol. 17, Solid Structures ofMacromolecules II, Kyoritsu Shuppan). The X-ray diffraction measurementconditions were as follows.

-   -   [X-ray diffractometer]: RINT2000 fully automatic X-ray        diffractometer made by Rigaku Corporation    -   Measurement method: Decker transmission method and Schulz        reflection method    -   Scanning speed: 40°/min    -   Scanning range:        -   transmission method 0.0 to 50.0°/10.0° step        -   reflection method 40.0 to 90.0°/10.00° step        -   Fixed 2θ angle: 16.0° (corresponds to 2θ        -   angle of β-crystal (300) plane)        -   X-ray: Cu/50 kv/200 mA            Porosity

The stretched film was cut into a square and the length of one side (Lcm), the weight (W g), and the thickness (D cm) were measured, and theporosity was calculated from the following equation.Porosity=100−100(W/ρ)/(L ² ×D)wherein ρ is the density of the unstretched polypropylene web sheetprior to stretching.Pore Size

The pore size was determined by the bubble point method (JIS K 3832), bymercury intrusion porosimetry, and by electron microscope (SEM)observation of a film cross section.

-   -   Bubble point (BP) method: The average pore size and maximum pore        size were measured using a bubble point type pore size        measurement apparatus (“Permporometer CFP-1200AEL” made by PMI).    -   Mercury intrusion porosimetry: Assuming that the pores were        cylindrical, their pore size was calculated from the following        equation using the total pore volume (V) and the pore specific        surface area (A) obtained from a mercury intrusion porosimetric        pore size measurement apparatus (Micromeritics AutoPore III        model 9420, made by Shimadzu Seisakusho).        Average pore size=4V/A    -   SEM observation: A porous film that had been cut to a size of 3        cm square was immersed in molten paraffin at 70° C., and the        film was impregnated with the paraffin until the film became        semitransparent. Then, the film was taken out and the paraffin        was cooled and solidified. The film was then thoroughly cooled        by bringing it into close contact with dry ice, and the film was        cut with a razor blade in the longitudinal and transverse        directions of the film. The impregnating paraffin was then        removed by extraction with hexane, and the film was dried. Gold        was deposited with an ion sputtering apparatus (Ion Sputter        JFC-1100 made by JEOL) to produce a film cross section        observation sample. This was placed under an electron microscope        (JSM-T200 made by JEOL), and micrographs were made of the film        cross section at a magnification of 1000 times to obtain cross        sectional images including the film surface. The maximum pore        size in the transverse direction, longitudinal direction, and        thickness direction were read from cross sectional images in the        transverse and longitudinal directions.        Gurley Air-permeability

The time (sec) it took for 10 ml of air to pass through a film surfacearea of 6.452 cm² under a pressure of 2.3cmHg was measured according toASTM D726.

Electrical Resistance

The electrical resistance per mil (25 μm) of film thickness wascalculated using (Formula 1) from the average pore size (μm) and theGurley air-permeability (sec) measured according to ASTM D726.

(Formula 1) consists of (Formula 2) and (Formula 3). The proportionalrelationship of (Formula 2) has been noted between the product (sec·μm)of the Gurley air-permeability (sec) and the average pore size (μm) andthe electrical resistance RmA (mohm·in²) (R. W. Callahan et al., TheTenth International Seminar on Primary and Secondary Battery Technologyand Application, Mar. 1–4, 1993), and the electrical resistance per milof film thickness is obtained from the resulting RmA and (Formula 3)(Japanese Unexamined Patent Publication No. 2000-30683).R=25(4.2t _(Gur) d)/L  (Formula 1)where R is the electrical resistance (ohm·in) per 25 μm of filmthickness in a 31 wt % KOH electrolyte solution, t_(Gur) is the Gurleyair-permeability (sec/10 ml) measured according to ASTM D726, d is theaverage pore size (μm) determined by mercury intrusion porosimetry, andL is the film thickness (μm)).

In the present invention, R value (ohm·in/mil) is used as the estimatedelectrical resistance. It is generally preferable that a batteryseparator have low electrical resistance. More specifically, theelectrical resistance R per mil of film thickness is less than 30ohm·in/mil, and preferably less than 20 ohm·in/mil.RmA=4.2t_(Gur)d  (Formula 2)R=25RmA/L  (Formula 3)

In the above (Formula 2) and (Formula 3),

-   -   RmA: estimated electrical resistance (mohm·in²) of the film in        31% KOH solution    -   t_(Gur): Gurley air-permeability (sec) measured according to        ASTM D726    -   d: average pore size (μm) determined by mercury intrusion        porosimetry    -   L: film thickness (μm)    -   R: estimated electrical resistance (ohm·in/mil) per 25 μm (1        mil) of film thickness in a 31 wt % KOH solution        Water Vapor Permeability

Measured according to JIS Z 0208.

Tensile Strength

Measured according to JIS K 7127.

Leakage Resistance

The water pressure resistance (kPa) was determined according to JIS L1092, except that a 0.25 wt % aqueous solution of a surfactant (sodiumpolyoxyethylene lauryl ether sulfate (number of moles of ethylene oxideadded: 3 moles)) was used instead of pure water.

Film Thickness Uniformity

The thickness of the obtained porous film was measured with a filmthickness meter (SME-1, made by SANKO ELECTRONIC LABORATORY CO., LTD.)at 100 points, with a 1 cm separation between points in the longitudinaldirection, along the center line in the width direction of the film(that is, the center line longitudinally connecting points that dividethe film width into two equal halves), the average thickness (Tave), themaximum thickness (Tmax), and the minimum thickness (Tmin) weredetermined, and the thickness uniformity was calculated from the formula(Tmax−Tmin)/Tave. The smaller this value, the higher the thicknessuniformity.

Hand (Feeling)

The obtained porous film was cut into a square measuring 30 cm on eachside, the film was balled in the palm of the hand, and its supplenesswas ranked according to the following three grades.

⊚: extremely good suppleness

◯: good suppleness

Δ: feeling of somewhat hard and stiff

Example A

[Method (I)]

N,N′-Dicyclohexyl-2,6-naphthalenedicarboxamide (0.05 weight part, usedas a β-crystal nucleating agent) and 0.05 weight part of Irgafos 168 and0.05 weight part of Irganox 1010 made by Ciba Specialty Chemicals (usedas antioxidants) were mixed in a Henschel mixer with 100 weight parts ofa propylene-ethylene block copolymer with an MFR of 2.7 g/10 minutes andan ethylene content of 6.2 wt %. This mixture was melt mixed at 250° C.in a single screw extruder, and the extruded resin was cooled and cut toprepare resin pellets containing the β-crystal nucleating agent. It wasconfirmed visually that the molten resin discharged from the die nozzleof the single screw extruder was transparent, indicating that theabove-mentioned amide compound had completely dissolved in the moltenpolypropylene during the melt mixing.

These resin pellets were then extruded in the form of a sheet at a resintemperature of 200° C. using a T-die extruder (twin screw extruder witha screw diameter of 65 mm, plus a T-die with a width of 350 mm). Thissheet was cooled and solidified by being placed for 12 seconds on achill roll with a diameter of 600 mm and maintained at a surfacetemperature of 12° C., giving an unstretched polypropylene web sheetwith a width of 300 mm and a thickness of 380 μm. It was confirmedvisually that the molten resin discharged from the T-die nozzle waswhite and semitransparent, indicating that the above-mentioned amidecompound had not completely dissolved during the T-die extrusion. Partof this unstretched web sheet (prior to longitudinal stretching) was cutout, and the K value and the β-crystal content were measured.

This sheet was then guided to a longitudinal stretching apparatus with aroll surface temperature of 90° C., where it was stretchedlongitudinally at a ratio of 4 times, giving a longitudinally stretchedsheet with a width of 255 mm. The distance between the longitudinalstretching rolls here was 100 mm, and the neck-in ratio in the widthdirection of the unstretched web sheet was 15%. After this longitudinalstretching, a sheet sample was cut out from the longitudinally stretchedsheet and subjected to X-ray diffraction measurement to determine thedegree of β-crystal orientation <cos²θ_(TD)>.

This longitudinally stretched sheet was then annealed while beinglongitudinally stretched at a stretch ratio of 10% with a roll having asurface temperature of 145° C. The annealing contact time during whichthe longitudinally stretched sheet was contacted with the roll was 5seconds.

This annealed sheet was then guided to a transverse stretchingapparatus, where it was subjected to transverse tenter stretching at aratio of 6.0 times at a temperature of 140° C. and a strain rate of100%/sec, whereby a white, translucent stretched film was continuouslyobtained.

Table 1 gives the manufacturing conditions, the various propertiesduring the manufacturing process, and the various properties of theobtained porous film.

Example B

[Method (I)+Method (II)]

A stretched film was obtained in the same manner as in Example A withthe exception of changing the distance between the longitudinalstretching rolls to 180 mm so as to adjust the neck-in ratio to 35%.Table 1 gives the various conditions and properties.

Example 1

[Method (II)]

A stretched film was obtained in the same manner as in Example A exceptthat the N,N′-dicyclohexyl-2,6naphthalenedicarboxamide used as aβ-crystal nucleating agent was used in an amount of 0.2 weight part, themolten resin temperature during resin pellet preparation was changed to240° C., the resin temperature during T-die extrusion was changed to220° C., and the distance between the longitudinal stretching rolls waschanged to 435 mm so as to adjust the neck-in ratio to 45%. Table 1gives the various conditions and properties.

The crystals of the above-mentioned β-crystal nucleating agent in thepellets obtained in this Example 1 were almost all columnar crystals,with some being needle crystals. The dissolution temperature of thesecrystals was approximately 280° C., and the β-crystals were present asprecipitated in a substantially unoriented state during the T-dieextrusion at 220° C.

Example 2

A stretched film was obtained in the same manner as in Example 1 withthe exception of changing the distance between the longitudinalstretching rolls to 230 mm so as to adjust the neck-in ratio to 35%.Table 1 gives the various conditions and properties.

Example 3

A stretched film was obtained in the same manner as in Example 1 withthe exception of changing the distance between the longitudinalstretching rolls to 155 mm so as to adjust the neck-in ratio to 25%.Table 1 gives the various conditions and properties.

Example 4

A stretched film was obtained in the same manner as in Example 1 withthe exception of omitting the roll annealing treatment after thelongitudinal stretching. Table 1 gives the various conditions andproperties.

Example 5

A stretched film was obtained in the same manner as in Example 2 withthe exception of omitting the roll annealing treatment after thelongitudinal stretching. Table 1 gives the various conditions andproperties.

Example 6

A stretched film was obtained in the same manner as in Example 1 exceptthat the longitudinal stretch ratio in the annealing step after thelongitudinal stretching was changed from 10% to 0%. Table 1 gives thevarious conditions and properties.

Example 7

A stretched film was obtained in the same manner as in Example 1 exceptthat the roll temperature in the annealing step after the longitudinalstretching was changed from 145° C. to 140° C. Table 1 gives the variousconditions and properties.

Example 8

A stretched film was obtained in the same manner as in Example 1 withthe exception of changing the transverse stretching strain rate to150%/sec. Table 1 gives the various conditions and properties.

Example 9

A stretched film was obtained in the same manner as in Example A withthe exception of using a propylene homopolymer with an MFR of 7.5 g/10minutes as the polypropylene-based resin, and changing the roll surfacetemperature during longitudinal stretching to 120° C. Table 1 gives thevarious conditions and properties.

Comparative Example 1

A stretched film was obtained in the same manner as in Example 1 withthe exception of changing the distance between the longitudinalstretching rolls to 115 mm so as to adjust the neck-in ratio to 15%.Table 1 gives the various conditions and properties.

Comparative Example 2

A stretched film was obtained in the same manner as in ComparativeExample 1 with the exception of omitting the roll annealing treatmentafter the longitudinal stretching. Table 1 gives the various conditionsand properties.

TABLE 1 Item Example Comp. Ex. Manufacturing conditions A B 1 2 3 4 5 67 8 9 1 2 Melt- Amount of β-crystal 0.05 0.05 0.2 0.2 0.2 0.2 0.2 0.20.2 0.2 0.05 0.2 0.2 mixing Nucleating agent (wt. part) Melt-kneadingtemp. (° C.) 250 250 240 240 240 240 240 240 240 240 250 240 240 Visualobservation of Trans- Trans- White, White, White, White, White, White,White, White, Trans- White, White, Molten resin parent parent turbidturbid turbid turbid turbid turbid turbid turbid parent turbid turbidDissolution temp. of 240 240 280 280 280 280 280 280 280 280 240 280 280Amide compound T-die Extrusion temp. (° C.) 200 200 220 220 220 220 220220 220 220 200 220 220 Extrusion Visual observation of White, White,White, White, White, White, White, White, White, White, White, White,White, Molten sheet translucent translucent turbid turbid turbid turbidturbid turbid turbid turbid trans- turbid turbid parent Web K value 0.960.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 Sheetβ-crystal content (%) 72 72 72 72 72 72 72 72 72 72 72 72 72 Longi- W:web sheet width (mm) 300 300 300 300 300 300 300 300 300 300 300 300 300tudinal L: distance between rolls 100 180 435 230 155 435 230 435 435435 100 115 115 stretching (mm) Neck-in ratio (%) 15 35 45 35 25 45 3545 45 45 15 15 15 Degree of β-crystal 0.26 0.24 0.26 0.28 0.30 0.26 0.280.26 0.26 0.26 0.26 0.32 0.32 Orientation <cos² θ _(TD)> Longitudinallystretched 255 195 165 195 225 165 195 165 165 165 255 255 255 sheetwidth (mm) Annealing Roll temperature (° C.) 145 145 145 145 145 No No145 140 145 145 145 No Time (sec) 5 5 5 5 5 Annealing Annealing 5 5 5 55 Annealing Longitudinal stretch ratio (%) 10 10 10 10 10 0 10 10 10 10Transverse Strain rate (%/sec) 100 100 100 100 100 100 100 100 100 150100 100 100 stretching Number of breakage 0 0 0 0 0 0 0 0 0 0 0 1 1 FilmThickness (μm) 43 60 68 55 45 63 50 65 64 64 44 37 34 propertiesPorosity (%) 57 60 58 56 54 55 52 56 56 55 57 50 45 Average BP method0.05 0.05 0.05 0.05 0.06 0.05 0.05 0.05 0.05 0.05 0.05 0.07 0.07 poresize Mercury intrusion 0.25 0.25 0.25 0.26 0.28 0.26 0.28 0.28 0.27 0.280.25 0.30 0.30 (μm) method Maximum BP method 0.055 0.055 0.055 0.0550.065 0.055 0.055 0.055 0.055 0.055 0.055 0.080 0.080 Pore sizeSEM/longitudinal direction/ 10 11 22 18 15 17 14 20 20 16 5 10 8 (μm)transverse direction/ 12 13 24 20 12 20 16 22 22 18 6 8 6 thicknessdirection 2.0 2.0 4.0 3.0 1.5 3.0 2.5 4.0 3.5 3.0 1.0 0.5 0.5 Gurley airpermeability/ASTM 16 10 12 20 32 32 48 16 20 16 10 100 240 (sec/10 ml)Estimated value of electrical resistance 9.8 4.4 4.6 10 21 14 28 7.2 8.97.4 6.0 85 220 (ohm · in/mil) Water vapor permeability (g/m² · 24 h)5000 5000 5000 4900 4800 4800 4400 5000 4900 4900 5000 2800 2000 TensileLongitudinal 75 73 70 74 76 72 74 70 70 69 90 77 74 strength Transverse80 76 75 78 80 76 78 74 74 76 75 80 80 (MPa) Water resistance pressure(KPa) 200–250 200–250 200–250 200–250 200–250 200–250 200–250 200–250200–250 200–250 200–250 200–250 200–250 Thickness uniformity 0.05 0.050.05 0.05 0.08 0.06 0.07 0.04 0.05 0.08 0.05 0.15 0.18 Hand (feeling) ⊚⊚ ⊚ ⊚ ◯ ⊚ ◯ ⊚ ⊚ ⊚ ◯ ◯ Δ

INDUSTRIAL APPLICABILITY

With the present invention, in the manufacture of a porous film composedof a stretched β-crystal-type polypropylene, it is possible to keep agood balance between the air permeability of the stretched film and itstendency toward breakage during stretching, which was a problem in thepast. This makes it possible to industrially produce under practicalconditions a porous polypropylene film with excellent air permeabilityand having continuous through pores, which can be applied as batteryseparators.

The film of the present invention is a porous polypropylene film whichhas excellent thickness uniformity, and high porosity and airpermeability, and which satisfies the electrical resistance required ofa battery separator.

1. A process for producing a successively biaxially stretched, β-crystalnucleating agent-containing polypropylene porous film, comprising apolypropylene-based resin and a β-crystal nucleating agent, the filmhaving a thickness uniformity of 0.1 or less, and the film exhibitingthe following pore structures (a) and (b) when observed in cross sectionin the longitudinal and transverse directions of the film under anelectron microscope: (a) in the cross section in the transversedirection: more lamella cross sections are present than in the image ofthe cross section in the longitudinal direction; there are numerouspores between these lamella cross sections; the maximum pore size in thethickness direction of the pores is 0.1 to 5 μm and the maximum poresize in the transverse direction is 1 to 50 μm; and the ratio of themaximum pore size in the thickness direction/the maximum pore size inthe transverse direction is from 1/2 to 1/20; (b) in the cross sectionin the longitudinal direction: there are no lamella cross sections orfewer lamella cross sections than in a cross sectional image in thetransverse direction; there are numerous pores; the maximum pore size inthe thickness direction of the pores is 0.1 to 5 μm; the maximum poresize in the longitudinal direction is 1 to 50 μm; and the ratio of themaximum pore size in the thickness direction/the maximum pore size inthe longitudinal direction is from 1/2 to 1/20, said process comprisinga sequential biaxial stretching step which comprises extruding a melt ofa polypropylene-based resin composition containing a β-crystalnucleating agent from a T-die, cooling the extrudate on a chill roll,and stretching the thus obtained β-crystal nucleating agent-containingpolypropylene unstretched web sheet first longitudinally and thentransversely, characterized in that the degree of orientation ofβ-crystals calculated from a pole figure of the crystal lattice (300)plane of the β-crystals determined by X-ray diffraction of the sheetobtained after longitudinal stretching is adjusted to less than 0.30 bycarrying out the following method (I) and/or method (II): method (I):melting a polypropylene-based resin composition consisting essentiallyof (i) the polypropylene-based resin, (ii) needle crystals of at leastone amide compound as the β-crystal nucleating agent, and if desired(iii) at least one polyolefin modifier at a temperature (T1) which isnot lower than the melting point of the polypropylene-based resin andlower than the temperature at which the needle crystals of the amidecompound dissolve in the melt of the polypropylene-based resin, andextruding the molten polypropylene-based resin composition from theT-die in a state in which the amide compound needle crystals arepresent, wherein said at least one amide compound is: (1) at least onemember selected from the group consisting of N,N′-diphenylhexanediamide,N,N′-dicyclohexylterephthalamide andN,N′-dicyclohexyl-2,6-naphthalenedicarboxamide, (2) at least one memberselected from the group consisting ofN,N′-dicyclohexanecarbonyl-p-phenylenediamine,N,N′-dibenzoyl-1,5-diaminonaphthalene,N,N′-dibenzoyl-1,4-diaminocyclohexane andN,N′-dicyclohexanecarbonyl-1,4-diaminocyclohexane, (3) at least onemember selected from the group consisting ofN-cyclohexyl-4-(N-cyclohexanecarbonylamino)benzamide andN-phenyl-5-(N-benzoylamino)pentaneamide, or (4) a mixture of two or moremembers of the above amide compounds of (1) to (3)), method (II):adjusting the neck-in ratio during longitudinal stretching to at least25% and not more than 55%.
 2. The process for producing a porous filmaccording to claim 1, wherein the sheet after the longitudinalstretching is annealed at 130 to 160° C. for 1 to 300 seconds whilebeing stretched in the longitudinal direction at a longitudinal stretchratio of 0 to 30%, and is then transversely stretched.
 3. The processfor producing a porous film according to claim 1 or 2, wherein, in thetransverse stretching step, the transverse stretching is performed at astretching temperature of 120 to 155° C. at a stretch ratio of 4 to 10times and at a transverse stretching strain rate of 100 to 300%/sec. 4.The process for producing a porous film according to claim 1, whereinsaid at least one polyolefin modifier is selected from the groupconsisting of stabilizers, UV absorbers, antioxidants, surfactants,lubricants, fillers, foaming agents, foaming auxiliaries, polymeradditives, plasticizers, crosslinking agents, crosslinking auxiliaries,antistatic agents, neutralizers, anti-blocking agents, anti-foggingagents, polymer alloy components, flame retardants, dispersants, organicand inorganic pigments and dyes, and working auxiliaries.